N-3 polyunsaturated fatty acids (PUFA) have anti-inflammatory effects and may be useful for the treatment of inflammatory diseases such as rheumatoid arthritis (RA).We examined the efficacy of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) supplementation on RA on top of standard anti-inflammatory treatment. Patients with RA were randomized into two groups in a double-blind, placebo-controlled, parallel-design multicenter study. One hundred nine patients received five capsules of either n-3 PUFA (2.090 g of EPA and 1.165 g of DHA) or high-oleic-acid sunflower oil for 16 weeks. Eighty-one patients completed the study, and no adverse effects were reported. Dietary intake did not change significantly during the study. There were significant increases in n-3 PUFA and EPA levels in erythrocytes in the n-3 PUFA group versus the placebo group, but decreases in n-6 PUFA, 18:2n6, 20:4n6 and 18:1n9 levels in the n-3 PUFA group versus the placebo group. N-3 PUFA supplementation had no significant effects on nonsteroidal anti-inflammatory drug (NSAID) requirements, clinical symptoms of RA or the concentration of cytokines, eicosanoids and bone turnover markers. However, n-3 PUFA supplementation significantly decreased NSAID requirements and leukotriene B4 levels in patients who weighed more than 55 kg. Our results suggest that n-3 PUFA supplementation has no significant effect on RA but may decrease the requirement for NSAIDs in Korean patients with RA who weigh more than 55 kg.