The effects of dietary supplementation with n-3 fatty acids on the level of cytokines and complement activation in plasma from patients with rheumatoid arthritis were examined. Thirty-two patients with active rheumatoid arthritis were included in a 12-week double-blind, randomized study of dietary supplementation with n-3 fatty acids (3.6 g per day) or placebo. The cytokines were measured in plasma before and after treatment with fish oil or placebo. In general, cytokine values at the upper limits of the calculated normal areas were found. The Interleukin-1 beta concentration in plasma was reduced significantly after 12 weeks of dietary supplementation with fish oil (p < 0.03). No significant difference was observed in the placebo group. The tumour necrosis factor alpha activity in plasma did not change significantly (p = 0.167). No significant changes were observed in the degree of complement activation. The clinical status of the patients was improved in the fish oil group, but not in the placebo group, judged by Ritchie's articular index (p < 0.02). We conclude that dietary supplementation with n-3 fatty acids results in significantly reduced plasma IL-1 beta levels in patients with rheumatoid arthritis. Even though the cytokine levels were low, the anti-inflammatory effect of n-3 fatty acids could in part be explained by their ability to decrease cytokine production.