To determine the efficacy of the new immunosuppressive agent ciamexon in patients with rheumatoid arthritis (RA), we conducted a 6‐month, prospective, double‐blind, placebo‐controlled study. The study included 21 outpatients with confirmed RA, who were randomized into 3 treatment groups of 7 patients each. Group 1 received 400 mg/day of ciamexon, group 2 received 100 mg/day of ciamexon, and group 3 received placebo. We investigated the influence of ciamexon on the clinical course, the systemic inflammatory activity, and the lymphocyte subsets in the peripheral blood. Significant, dose‐dependent improvement was seen in both the clinical and the biochemical activity indexes at the end of the treatment period (P = 0.02 to P = 0.05). The proportion of activated T lymphocytes was significantly decreased (P = 0.05), and the proportion of CD8‐positive lymphocytes was significantly increased (P = 0.03) in patients taking ciamexon. The major adverse effects were hepatotoxicity (2 patients) and rash (2 patients). This study documents the clinical efficacy of ciamexon therapy in RA patients and identifies the agent's potential toxicity. Copyright © 1990 American College of Rheumatology