An encapsulated preparation of fish oil (Maxepa) was administered to hyperlipidaemic patients in order to establish the responsiveness of the common lipid phenotypes to dietary supplementation with n-3 fatty acids. 13 patients took 6 g/day of fish oil and 12 patients took 16 g/day in a randomized, double-blind crossover study, whereby each subject took fish oil for 3 months and matching placebo for 3 months. The study was conducted against a background diet restricted in saturated fat and cholesterol. In Types IIa and IIb hyperlipoproteinaemia there was no substantial fall in plasma cholesterol concentration. Plasma triglyceride concentrations were reduced significantly in Types IIb and IV (28% and 41% respective reductions). In a separate study using 16 g/day of fish oil in patients with Type V hyperlipoproteinaemia, plasma triglycerides were reduced by 58% and plasma cholesterol concentration by 34%. The change in plasma triglyceride concentration was significantly correlated with the basal triglyceride level (r = -0.94), and was dose-related (33% fall on 6 g/day and 58% fall on 16 g/day). The fall in plasma triglyceride concentration was accompanied by a significant reduction in the concentration of very low-density lipoprotein cholesterol (-42%), a significant rise in low density lipoprotein cholesterol (+7%), and a significant rise in high-density lipoprotein cholesterol concentration (+6%), there being no significant change in the ratio of low density to high density lipoprotein cholesterol. There were changes in the fatty acid composition of plasma and platelet lipids which reflected dietary supplementation with n-3 fatty acids, notably an increase in the proportion of eicosapentaenoic and docosahexaenoic acids which occurred in a dose-dependent fashion. Despite these changes there was no significant variation in the bleeding time, platelet count or blood viscosity during the treatment.