In August 2025, the Ninth Circuit affirmed a district court’s decision to exclude an expert’s causation opinion as unreliable and grant summary judgment in favor of a herbicide manufacturer. The case, which arose from claims that exposure to an herbicide caused the plaintiff’s blood cancer, underscores the critical importance of rigorous and well-supported expert analysis in toxic tort litigation and the judiciary’s gatekeeping role under Federal Rule of Evidence 702.¹

1. Background: The Litigation and the Federal Rules on Expert Testimony

The underlying lawsuit was filed in 2019, with the plaintiff, alleging that he was diagnosed with chronic lymphocytic leukemia (CLL), a form of blood cancer, in 2014 as a result of years of using the defendant’s herbicide. The admissibility of expert testimony in federal court is governed by Federal Rule of Evidence 702.² Rule 702 requires that “before admitting expert testimony, the district court must perform a gatekeeping role to ensure that the [proffered] testimony is both relevant and reliable.”³ The Supreme Court’s decision in Daubert v. Merrell Dow Pharmaceuticals, Inc. clarified that trial courts must assess not only the qualifications of the expert but also the reliability of the methodology used.⁴ Daubert emphasized that mere credentials or experience are not enough – the  expert’s methods must be scientifically valid and properly applied to the facts of the case.  The Court also discussed the 2023 amendment to Federal Rule of Evidence 702, which expressly requires “a proponent of expert testimony to ‘demonstrate to the court that it is more likely than not that’” the expert’s testimony satisfies the admissibility requirements of Rule 702.⁵

2. Plaintiff’s Causation Theory and the Expert’s Differential Etiology

To support his claim, Plaintiff relied on a board-certified oncologist who conducted a “differential etiology”—an established scientific technique for determining the cause of a medical condition by systematically ruling in all of the potential causes of a plaintiff’s disease and then ruling out those possible causes to reach a conclusion as to the most likely cause of a plaintiff’s disease.⁶ In toxic tort litigation, a plaintiff must show both general causation (that the chemical is capable of causing the alleged injury) and specific causation (that the exposure actually caused the plaintiff’s injury).⁷ The oncologist in this case submitted an expert report addressing both general and specific causation, with specific causation being the central issue on appeal.

The expert’s report concluded that exposure to the defendant’s herbicide was a substantial factor in causing Plaintiff’s CLL. However, with respect to a potential alternative cause of Plaintiff’s disease, the expert’s report stated only that Plaintiff was “negative” for obesity, based solely upon unverified information Plaintiff provided informally in the case, without citing medical records, BMI, or other evidence. The expert’s report did not discuss whether obesity or other identified conditions could contribute to CLL, and did not cite to any literature ruling out obesity as a cause of plaintiff’s disease.

3. District Court’s Ruling

The defendant moved to exclude the expert’s opinion, arguing that it was not based on sufficient facts or data and failed to reliably rule out obesity as a potential cause. During cross-examination, the expert conceded he had not examined Plaintiff and could not say whether the was obese or not. He maintained that, in his clinical experience, obesity does not contribute to CLL, even though some medical literature reports an association between obesity and this type of cancer.

After the hearing, the District Court found that the expert’s differential etiology was unreliable under Rule 702 and excluded his causation opinion. Because Plaintiff had no admissible expert testimony on causation, the court granted summary judgment for the defense.

4. The Ninth Circuit’s Holding

On appeal, the Ninth Circuit affirmed the district court’s decision. The appellate court emphasized that there is no presumption in favor of admitting expert testimony; the proponent must establish admissibility by a preponderance of the evidence.⁸ While differential etiology is an accepted method, it must be executed rigorously.

Here, there were two fatal flaws in the expert’s testimony.  Initially, the expert’s testimony was not based “on sufficient facts or data.”⁹ The expert had neither examined the plaintiff, nor considered his medical records or deposition testimony as it related to the Plaintiff’s weight and therefore conceded he could not say whether the plaintiff was or was not obese. Thus, there was no basis for his opinion that the plaintiff was “negative” for obesity.¹⁰

Moreover, the expert’s report did not discuss whether obesity was recognized as a risk factor for the plaintiff’s cancer, did not address the literature on obesity, and had done no analysis to rule out obesity as a cause of Plaintiff’s cancer as required in a differential etiology.¹¹ Finally, the expert’s efforts to offer new rationale during cross-examination was inadmissible because it was not disclosed in the expert report.¹²

5. Looking Forward: Implications for Expert Testimony

The Ninth Circuit’s decision in this action is a reminder that courts will closely scrutinize the foundation for expert opinions in toxic tort and product liability cases, especially in light of the 2023 amendments to Rule 702. Parties to litigation must ensure their experts not only apply accepted methodologies such as differential etiology, but also rigorously document and explain their reasoning, including the facts and data relied upon and how they ruled out alternative causes. Experts cannot rely solely on credentials or clinical experience; they must tie their opinions to facts, data, and scientific literature. Failure to meet Rule 702’s reliability requirements can be fatal to both the expert’s testimony and the entire case at the summary judgment stage. As courts continue to enforce strict gatekeeping standards, parties should expect heightened scrutiny of expert declarations supporting or opposing dispositive motions.

1. Engilis v. Monsanto Company (2025) 151 F.4th 1040. ↩︎
2. Fed. R. Evid. 702; see also Elosu v. Middleford Ranch Inc., 26 F.4th 1017, 1023 (9th Cir. 2022). ↩︎
3. United States v. Valencia-Lopez, 971 F.3d 891, 897–98 (9th Cir. 2020). ↩︎
4. Daubert v. Merrell Dow Pharm., Inc., 509 U.S. 579, 589–97 (1993). ↩︎
5. Engilis, 151 F.4th at 1049. ↩︎
6. Engilis, 151 F.4th at 1044-45; Fed. Jud. Ctr., Reference Manual on Scientific Evidence 617 (3d ed. 2011). ↩︎
7. Golden v. CH2M Hill Hanford Grp., 528 F.3d 681, 683 (9th Cir. 2008). ↩︎
8. Engilis, 151 F.4th at 1050. ↩︎
9. Id. at 1051. ↩︎
10. Id. ↩︎
11. Id. at 1050. ↩︎
12. Fed. R. Civ. P. 26(a)(2)(B). ↩︎