Description: Background Patients with peripheral artery disease (PAD) experience significant morbidity and mortality. The OMEGA-PAD I Trial, a randomized, double-blinded, placebo-controlled trial, addressed the hypothesis that short-duration, high-dose n-3 polyunsaturated fatty acids (n-3 PUFA) oral supplementation improves endothelial function and inflammation in PAD. Methods and Results Eighty patients with stable claudication received 4.4 g of fish oil or placebo for 1 month. The primary end point was endothelial function as measured by brachial artery flow-mediated vasodilation. Secondary end points included biomarkers of inflammation, n-3 polyunsaturated fatty acids metabolome changes, lipid profile, and walking impairment questionnaires. Although there was a significant increase in FMD in the fish oil group following treatment (0.7±1.8% increase from baseline, P =0.04), this response was not different then the placebo group (0.6±2.5% increase from baseline, P =0.18; between-group P =0.86) leading to a negative finding for the primary endpoint. There was, however, a significant reduction in triglycerides (fish oil: –34±46 mg/dL, P 〈0.001; placebo –10±43 mg/dL, P =0.20; between-group differential P -value: 0.02), and an increase in the omega-3 index of 4±1% ( P 〈0.001) in the fish oil group (placebo 0.1±0.9%, P =0.49; between-group P 〈0.0001). We observed a significant increase in the production of pathway markers of specialized pro-resolving mediators generated from n-3 polyunsaturated fatty acids in the fish oil group. Conclusions High-dose, short-duration fish oil supplementation did not lead to a different response in the primary end point of endothelial function between the treatment and placebo group, but improved serum triglycerides and increased the production of downstream n-3 polyunsaturated fatty acids–derived products and mediators in patients with PAD. Clinical Trial Registration URL: https://www.clinicaltrials.gov/ . Unique identifier: NCT01310270.