Primary biliary cirrhosis is a progressive disease of the liver characterized by the immunologic destruction of bile ducts; effective therapy is lacking. We therefore evaluated the safety and efficacy of low-dose cyclosporine in 29 patients with primary biliary cirrhosis without evidence of damage to the lobular architecture (precirrhotic disease) or portal hypertension. The patients were randomly assigned to receive either cyclosporine (4 mg per kilogram of body weight per day) or placebo. After one year 17 of the 19 patients assigned to cyclosporine had improvement or stability in their degree of fatigue, and 18 in their degree of pruritus. In contrast, among the 10 patients assigned to placebo, fatigue increased in 4 (P less than 0.06) and pruritus worsened in 6 (P less than 0.001). Those assigned to cyclosporine also had significant decreases in serum levels of bilirubin, alanine aminotransferase, alkaline phosphatase, gamma globulin, and the titer of antimitochondrial antibodies. For the 20 patients who have completed two years in the study, liver biopsies (coded specimens) showed evidence of histologic progression in only 1 of 13 patients in the cyclosporine group, as compared with 5 of 7 in the placebo group (P less than 0.003). No patient has permanently discontinued cyclosporine because of side effects; however, signs of nephrotoxicity developed in 12 of 19, and 9 of 19 had increased blood pressure. We conclude that in patients with precirrhotic primary biliary cirrhosis, immunosuppressive therapy with cyclosporine is promising and deserves further evaluation.