The effects of dietary supplementation with selenium were studied in 6 patients with severe, active rheumatoid arthritis (RA) and in 6 healthy control subjects. Initial concentrations of Se in red blood cells and in serum, and the activity of the Se-dependent enzyme glutathione peroxidase (GSH-Px) in red blood cells, serum, and granulocytes were significantly lower in RA patients compared with controls. During Se supplementation, however, the differences in Se levels and in GSH-Px activity between the 2 groups disappeared, except that, in RA patients, GSH-Px activity in granulocytes increased but remained significantly lower than in controls.