The efficacy of cyclosporin in a double blind, placebo controlled trial of four months' duration has previously been reported by us. To assess the benefit-to-risk of cyclosporin this study was followed by a one year open trial including 49 of the previous 52 patients. Cyclosporin was given at an initial dosage of 5 mg/kg/day and then modulated on the basis of efficacy and renal toxicity. During the study the drug had to be discontinued in 32 patients: because of inefficacy in 10, side effects in 11, both in nine, and in two because of unrelated events. The significant clinical improvement noted at four months persisted through one year in the 17 patients who continued to receive treatment. Nephrotoxicity of the drug (24 patients) required constant and close monitoring as well as modulation of daily drug dosage during the trial. This study indicated that cyclosporin might be valuable in the treatment of patients with advanced rheumatoid arthritis, in whom other second line agents have failed.