IMPORTANCE: In patients treated with ω-3 fatty acids, it remains uncertain whether achieved levels of eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) are associated with cardiovascular outcomes. OBJECTIVE: To determine the association between plasma levels of EPA and DHA and cardiovascular outcomes in a trial of ω-3 fatty acids compared with corn oil placebo. DESIGN, SETTING, AND PARTICIPANTS: A double-blind, multicenter trial enrolled patients at high cardiovascular risk with elevated triglyceride levels and low levels of high-density lipoprotein cholesterol at 675 centers (enrollment from October 30, 2014, to June 14, 2017; study termination January 8, 2020; last visit May 14, 2020). INTERVENTIONS: Participants were randomized to receive 4 g daily of ω-3 carboxylic acid (CA) or an inert comparator, corn oil. MAIN OUTCOMES AND MEASURES: The primary prespecified end point was a composite of cardiovascular death, nonfatal myocardial infarction, nonfatal stroke, coronary revascularization, or unstable angina requiring hospitalization. The primary outcome measure was the hazard ratio, adjusted for baseline characteristics, for patients treated with the ω-3 CA compared with corn oil for the top tertile of achieved EPA and DHA plasma levels 12 months after randomization. RESULTS: Of the 13 078 total participants, 6539 (50%) were randomized to receive ω-3 CA and 6539 (50%) randomized to corn oil. ω-3 Fatty acid levels were available at both baseline and 12 months after randomization in 10 382 participants (5175 ω-3 CA patients [49.8%] and 5207 corn oil-treated patients [50.2%]; mean [SD] age, 62.5 [8.9] years, 3588 [34.6%] were women, 9025 [86.9%] were White, and 7285 [70.2%] had type 2 diabetes). The median plasma levels at 12 months in ω-3 CA patients were 89 μg/mL (interquartile range [IQR], 46-131 μg/mL) for EPA and 91 μg/mL (IQR, 71-114 μg/mL) for DHA with top tertile levels of 151 μg/mL (IQR, 132-181 μg/mL) and 118 μg/mL (IQR, 102-143 μg/mL), respectively. Compared with corn oil, the adjusted hazard ratios for the highest tertile of achieved plasma levels were 0.98 (95% CI, 0.83-1.16; P = .81) for EPA, and 1.02 (95% CI, 0.86-1.20; P = .85 for DHA. Sensitivity analyses based on changes in plasma and red blood cell levels of EPA and DHA and primary and secondary prevention subgroups showed similar results. CONCLUSIONS AND RELEVANCE: Among patients treated with ω-3 CA, the highest achieved tertiles of EPA and DHA were associated with neither benefit nor harm in patients at high cardiovascular risk. TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT02104817.