A prospective study was performed to evaluate the outcome of treatment withdrawal in 30 patients with "autoimmune" chronic active hepatitis in remission for periods of 1.5 to 9 years on maintenance corticosteroid and azathioprine therapy. Reactivation of disease, with marked rises in serum aminotransferase level (mean 668 +/- S.D. 458 IU per liter) and accompanied by severe symptoms, occurred in 25 (87%) patients within 52 weeks (median 9 weeks; range 5 to 52) and was associated with the histological features of piecemeal necrosis and lobular hepatitis in all 20 liver biopsies examined. Age, sex, duration of disease and remission, presence of cirrhosis, autoantibody status, or immunoglobulin levels did not differentiate patients who relapsed from those who remained in remission. The response to reinstitution of treatment with prednisolone was satisfactory in 25 patients and clinical and biochemical abnormalities resolved within 10 weeks (median 6; range 3 to 10), death occurred in one patient within 48 hr of readmission to hospital.