OBJECTIVES--Oxygen free radicals have been implicated as mediators of tissue damage in patients with rheumatoid arthritis (RA). Thus it is possible that several micronutrients acting as antioxidants and free radical scavengers provide protection against RA. Serum alpha-tocopherol, beta-carotene, and selenium were studied for their associations with the risk of RA. METHODS--A case control study was nested within a Finnish cohort of 1419 adult men and women. During a median follow up of 20 years, 14 individuals initially free of arthritis developed RA. Two controls per each incident case were individually matched for sex, age, and municipality. Serum alpha-tocopherol, beta-carotene and selenium concentrations were measured from stored serum samples. An antioxidant index was calculated as the product of the molar concentrations of these three micronutrients. RESULTS--Elevated risks of RA were observed at low levels of alpha-tocopherol, beta-carotene and selenium, but none of the associations were statistically significant. A significant association, however, was observed with a low antioxidant index (p for trend = 0.03), the relative risk of RA between the lowest tertile and the higher tertiles of its distribution being 8.3 (95% confidence interval 1.0-71.0). CONCLUSIONS--The results of the present study are in line with the hypothesis that a low antioxidant level is a risk factor for RA.