A study was carried out to evaluate the clinical and haematological effects of dietary supplementation with eicosapentaenoic acid (EPA)-rich fish oil (MaxEPA', 2.8 g EPA daily) compared to placebo (olive oil) in 10 patients with stable angina pectoris. After 3 months, there was a significant increase in red cell deformability (p less than 0.001), reduced whole blood viscosity (p less than 0.02), and prolonged skin bleeding time (p less than 0.001) in the fish oil group compared to the placebo group. Haematocrit, plasma viscosity, fibrinogen concentration, platelet count, and in vitro platelet aggregation were unaltered. No significant symptomatic or objective improvement was noted in angina pectoris in either group despite the significant rheological changes produced in the patients receiving fish oil.