Rebuttal Reports

What the rule authorizes, and the two words that limit it

The rebuttal disclosure has no separate rule of its own. It exists inside the timing provision, at FRCP 26(a)(2)(D)(ii), which sets the deadline for evidence

“intended solely to contradict or rebut evidence on the same subject matter identified by another party under Rule 26(a)(2)(B) or (C).”

Both qualifiers are load-bearing. Solely excludes a report that does two jobs, and same subject matter excludes a report that answers something the opposing expert did not address. Neither is a drafting nicety; between them they account for most of what gets struck.

Two further points about what the subparagraph is and is not. It sets a time, not a reduced content standard: where the rebuttal witness is retained, the rebuttal submission must satisfy all of FRCP 26(a)(2)(B), including the ten-year publication list, the four-year testimony list and the compensation statement. And the 30-day figure is a default that applies only absent a stipulation or court order, which in a case with a scheduling order it usually is not.

The published circuit formulation

District courts policing rebuttal reports quote a decision about rebuttal testimony at trial. Marmo v. Tyson Fresh Meats, Inc., 457 F.3d 748 (8th Cir. 2006), supplies both sentences that recur:

“The function of rebuttal testimony is to explain, repel, counteract or disprove evidence of the adverse party.”

And rebuttal evidence serves to

“challenge the evidence or theory of an opponent — and not to establish a case-in-chief.”

The second sentence is the one that answers the affirmative-opinion question, and it comes from a published court of appeals decision rather than from a district court gloss.

What Marmo actually held is narrower than the quotations suggest, and it is worth stating accurately. The Eighth Circuit found no abuse of discretion in the district court's refusal to let a physician testify as a rebuttal witness, because the defendant had presented no evidence on medical causation for him to rebut, and a jury instruction cured any prejudice from opposing counsel's opening statement. The standard of review was abuse of discretion, the allowance of rebuttal evidence resting with the trial judge as the person best placed to weigh the competing circumstances. The case addresses rebuttal testimony at trial rather than a Rule 26(a)(2)(D)(ii) report; it is nonetheless the formulation courts apply to reports. Opinion at openjurist.

New data and new methods are permitted; new theories are not

The most useful recent statement of the standard is In re Uber Technologies, Inc., Passenger Sexual Assault Litigation (N.D. Cal. Dec. 17, 2025) (Cisneros, M.J.), which collects the authorities before applying them. Two of the quoted propositions do most of the work in practice, and they point in opposite directions.

From Wadler v. Bio-Rad Laboratories, Inc. (N.D. Cal. Oct. 17, 2016): “Rebuttal testimony cannot be used to advance new arguments or new evidence.” And, more precisely, “[t]he test of whether an expert's opinion constitutes rebuttal or a 'new' opinion … is not whether a rebuttal expert employs new testing or methodologies but instead, whether a rebuttal attempts to put forward new theories outside the scope of the report it claims to rebut.”

From In re MacBook Keyboard Litigation (N.D. Cal. Jan. 25, 2022): “A rebuttal expert may cite new evidence and data or introduce new methods of analysis in a rebuttal report so long as the new evidence, data, or method is offered to contradict or rebut the opposing party's expert.”

Read together, the line is drawn at purpose rather than at novelty. A rebuttal expert who runs a different model to show that the opposing expert's model is sensitive to an assumption is contradicting; the same model run to establish the party's own damages figure is a case-in-chief opinion served late. The distinction between a new method and a new theory does real work, and it is not applied uniformly — some courts treat any new analysis in a rebuttal report as an affirmative opinion. That divergence is worth knowing before the report is written rather than after the motion is filed.

What a court does when a rebuttal report goes too far

The ordinary remedy is surgical rather than total. In the December 2025 Uber decision, the court struck the portions of one rebuttal report that strayed beyond the scope of the report being rebutted — including three affirmative opinions praising the defendant's own risk-identification efforts — while leaving intact the opinions that directly contradicted the opposing expert. The same order declined to strike a separate supplemental report that rested on deposition material which became available after the original disclosure, ordering one additional hour of deposition as the cure.

The framework the court applied, drawn from United States v. Austin Radiological Ass'n (W.D. Tex. June 4, 2014), is three steps and it is reproducible: identify the evidence the rebuttal expert purports to contradict; ask whether the rebuttal is on the same subject matter; and ask whether it is intended solely to rebut.

Where the answer to the third question is no across the whole document, courts strike the whole document. In McCann v. Ogle County (N.D. Ill. Oct. 5, 2016), the court struck both of the plaintiff's rebuttal expert reports as improperly buttressing the case in chief and introducing new opinions, holding that a party “may not offer testimony under the guise of 'rebuttal' only to provide additional support for his case in chief.”

The diagnostic that predicts the ruling

The case law reduces to a single question that can be asked of every opinion in a draft rebuttal report: would this opinion have been necessary if the opponent had served no report at all?

If the answer is yes, the opinion is affirmative and belonged in the opening disclosure. That is why causation and damages opinions are the ones most often struck out of rebuttal reports — they are elements the disclosing party has to prove, and an element a party must prove does not become rebuttal because the other side's expert also discussed it.

Two corollaries follow, and both appear in the decisions.

• Citing the opposing expert does not convert an affirmative opinion. A report that makes a few references to the other side's expert is not thereby proper rebuttal; the references have to be what the opinion is doing, not decoration on an opinion that stands alone.
• Bolstering is not rebutting. Additional support for a conclusion the party already advanced is the paradigm case of an opinion that should have been disclosed with the opening report.

What a rebuttal report may do is correspondingly clear. It may attack the opposing expert's methodology, assumptions, data selection, calculations and conclusions; it may show that the method was not applied to these facts; it may show that an alternative explanation was never excluded; and it may deploy new data or a new analytical technique for the purpose of demonstrating any of those things.

There is no right to the last word

Rule 26(a)(2)(D)(ii) provides for rebuttal. It says nothing about sur-rebuttal, and courts have declined to read one in. In Wavetronix LLC v. Iteris, Inc., No. 6:21-cv-00899 (W.D. Tex. Jan. 4, 2024), the court struck sur-rebuttal expert reports and rejected a reading of the rule that would create a never-ending right to file rebuttal reports.

Where a party genuinely needs to respond to something new in a rebuttal report, the route is a motion for leave or a stipulation, not a unilateral service. That is a scheduling question for the court, and it is answered on the same considerations that govern any late disclosure — what is new, why it could not have been anticipated, and what accommodation cures the prejudice.

What happens to an opinion that is struck

An opinion excluded from a rebuttal report as affirmative does not usually get a second chance under another label. FRCP 26(e) permits correction of a disclosure that is incomplete or incorrect; the settled gloss is that it does not license new opinions, new methodologies or a strengthened analysis served after the deadline, and courts apply FRCP 37(c)(1) preclusion where it is used that way. The opinion belonged in the opening report, and the deadline for the opening report has passed.

The realistic sequence is therefore front-loaded. Every opinion the disclosing party must prove goes in the opening disclosure, including the ones the party expects the opponent to attack, because an opinion held back to answer the anticipated attack is the opinion that gets struck. The rebuttal report is then genuinely responsive, which is both what the rule requires and what makes it hard to challenge.

Where the material is genuinely new — a deposition taken after the report, data produced late, an analysis the opposing expert performed for the first time in their own report — the position is different, and courts have been willing to allow the response with a cure short of exclusion. The distinction the decisions draw is between a party responding to something that changed and a party improving a case it could have made on time.

Frequently Asked Questions

Published 2026-08-12