The efficacy and safety of cyclosporin A (CyA) for patients with rheumatoid arthritis were assessed in a four month double blind, placebo controlled study using an initial dosage of 5 mg/kg daily. Six patients withdrew from the study (two in the placebo group because of inefficacy of treatment and four in the CyA group because of side effects). These six patients were considered therapeutic failures. At the end of the trial the study treatment was considered as good or very good by 14 out of the 26 CyA group patients and by only two out of the 26 placebo group patients. Moreover, in the CyA group significant improvement was observed in five of the seven clinical assessment criteria. Clinical improvement was correlated with a decrease in C reactive protein, alpha 1 glycoprotein levels, and platelet count but not with erythrocyte sedimentation rate or rheumatoid factor titres. Renal toxicity (13 cases) remained the major problem in the management of these patients. This study shows that CyA is effective in active rheumatoid arthritis but requires close monitoring for toxicity.