We studied free radical generation, lipid peroxidation and the levels of essential fatty acids and of their metabolites in patients with rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). Both superoxide and hydrogen peroxide generation by peripheral leukocytes but not malondialdehyde levels, as measured by thiobarbituric acid assay, were found to be significantly enhanced both in RA and SLE. Fatty acid analysis of the plasma PL fraction revealed that both LA and ALA metabolites are significantly decreased in RA and SLE compared to controls. These results suggest that essential fatty acid metabolism is altered in RA and SLE.