To assess the nature and prognosis of severe chronic active hepatitis of unknown cause, we compared 26 patients who had been fully screened for etiologic factors with 112 patients who had autoimmune chronic active hepatitis after similar durations of corticosteroid therapy (17(+)/- 2 versus 23 (+)/- 2 months), and follow-up versus 103 +/- 7 months). Patients with cryptogenic disease could not be distinguished from those with autoimmune disease on the basis of age, sex distribution, duration of illness, immunoglobulin levels, frequency of concurrent immunologic disorders, or histologic findings. Serum gamma-globulin levels were higher (3.4 +/- 0.1 versus 2.5 +/- 0.2 g/dl, P = 0.007) and albumin levels were lower (2.9 +/- 0.1 versus 3.3 +/- 0.1 g/dl, P = 0.003) in patients with autoimmune disease than in those with cryptogenic disease, but individual findings did not differentiate the patients. Remission (69 versus 75%), treatment failure (23 versus 13%), relapse after drug withdrawal (67 versus 68%), progression to cirrhosis (57 versus 36%), and death from hepatic failure (12 versus 11%) occurred as commonly in patients with cryptogenic as in those with autoimmune disease. Patients with different constellations of immunoserologic findings were similar clinically. We conclude that patients with severe chronic active hepatitis who have been fully screened for etiologic factors cannot be distinguished from patients with autoimmune disease of comparable severity. These two groups of patients have a similar prognosis after corticosteroid therapy, and such treatment should be considered in these highly selected patients.