A controlled, double blind, parallel group, long term study of hydroxychloroquine sulphate in the treatment of rheumatoid arthritis, comparing daily doses of 200 mg and 400 mg, is described. The trial involved 54 patients with moderate disease activity who had not previously received antimalarial drugs. Forty three patients completed the one year treatment. The groups receiving different doses were homogeneous and did not differ in any of the 25 monitored indicators. Both dose regimens were effective, and a significant reduction of disease activity was observed after one year's treatment. Of the nine laboratory and 11 clinical indices of efficacy monitored, no statistically significant differences were reported, but in the group of patients treated with the 400 mg daily dose the number of side effects was three times greater. As there have been no reports of retinopathy with hydroxychloroquine at daily doses of 200 mg the effectiveness of this dose is of practical importance.