Epidemiologic and experimental data suggest that a high dietary intake of long-chain polyunsaturated n-3 fatty acids may reduce the risk of atherothrombotic disease. In a randomized, controlled study, 610 patients undergoing coronary artery bypass grafting were assigned either to a fish oil group, receiving 4 g/day of fish oil concentrate, or to a control group. All patients received antithrombotic treatment, either aspirin or warfarin. Their diet and serum phospholipid fatty acid profiles were monitored. The primary end point was 1-year graft patency, which was assessed by angiography in 95% of patients. Vein graft occlusion rates per distal anastomoses were 27% in the fish oil group and 33% in the control group (odds ratio 0.77, 95% confidence interval, 0.60 to 0.99, p = 0.034). In the fish oil group, 43% of the patients had > or = 1 occluded vein graft(s) compared with 51% in the control group (odds ratio 0.72, 95% confidence interval, 0.51 to 1.01, p = 0.05). Moreover, in the entire patient group, there was a significant trend to fewer patients with vein graft occlusions with increasing relative change in serum phospholipid n-3 fatty acids during the study period (p for linear trend = 0.0037). Thus, in patients undergoing coronary artery bypass grafting, dietary supplementation with n-3 fatty acids reduced the incidence of vein graft occlusion, and an inverse relation between relative change in serum phospholipid n-3 fatty acids and vein graft occlusions was observed.