Between 1968 and 1974, azathioprine has been used in a controlled prospective trial to treat patients with symptomatic but precirrhotic primary cirrhosis. Forty-five patients were admitted, of whom 22 were given azathioprine in a dose of 2 mg per kg of body weight. During the 1st year, serum aspartate transaminase levels showed a significant change in favor of the treated group, but improvement did not continue. Throughout the trial, serum alkaline phosphatase, bilirubin, cholesterol, albumin and immunoglobulin M values showed no significant change. Titers of serum mitochondrial antibodies tended to become negative more often in the treated than the untreated. Pruritus cannot be assessed objectively, but seemed less in the treated than in controls. Serial hepatic biopsy specimens showed the development of cirrhosis equally in the two groups. Survival, as judged by the life table method, was similar for the first 5 years of the trial. There was, however, a significant difference in favor of the treated group in the 6th year, although the number of patients available for assessment at that time was extremely small.