As fish oil has been shown to be beneficial in rheumatoid arthritis and in psoriasis, we examined whether a diet rich in fish has a similar effect on membrane and plasma lipids as a dietary fish oil supplement. Volunteers recruited by 2 rheumatology units in Switzerland formed three parallel groups eating respective diets during 2 months: a normal diet without fish; a normal diet including 700 g fish per week; a normal diet without fish but with additional fish oil (7.5 g daily). As outcome measures we determined the lipid composition of platelet-rich plasma, the serum cholesterol and triglycerides before the study and after 1 and 2 months of the designated diet. The relative amounts of both eicosapentaenoic acid and docosahexaenoic acid increased significantly in the fish oil group and in the group with the fish diet; no change was seen in the control group. The effect on triglycerides, which were low at the beginning of the study, was minor and no change in cholesterol was seen. In conclusion, 4 to 6 meals with fish per week without any other dietary changes can induce similar changes in lipids as a supplement of fish oil.