To examine the potential role of fish oil supplementation in the prevention of restenosis after coronary angioplasty (PTCA), a randomised double-blind trial was conducted in 204 patients. The treatment group received 6 g/day of n-3 fatty acids, beginning 5.4 (SD 3.2) days before PTCA, and continuing for 6 months; the control group received olive oil placebo. Compliance was assessed by pill count and plasma levels of eicosapentaenoic acid (EPA). Restenosis was identified by symptoms and exercise testing and confirmed by angiography. PTCA was successful in 186 patients (93%). The incidence of angiographic restenosis was 34% in the fish oil group and 23% in the control group (relative risk 1.7, 95% CI 0.9-3.4). The lack of benefit of fish oil was not influenced by length of pretreatment, compliance with study medication, or the concentrations of plasma EPA achieved.