To determine the nature of unexplained chronic serum aspartate aminotransferase elevations of a mild to moderate degree in asymptomatic patients, we performed systematic clinical, biochemical and histologic examinations in 47 individuals who had been screened for virus-, alcohol-or drug-related disease. Serum aspartate aminotransferase levels ranged from 3-to 8-fold normal (mean: 156 ± 7 units per liter) for at least 6 months (mean: 30 ± 6 months). Serum alanine aminotransferase levels were also increased but to a lesser degree in most patients. Thirty-four patients (72%) had histologic features of chronic active hepatitis, including 16 with cirrhosis. Ten patients (21%) had steatohepatitis and three (6%) had miscellaneous disorders. Patients with chronic active hepatitis and cirrhosis could not be distinguished from counterparts without cirrhosis by individual clinical or laboratory findings. Antinuclear or smooth muscle antibodies were detected in 18 of the patients with chronic active hepatitis (53%). All patients with steatohepatitis were women, and they had laboratory changes at presentation, including seropositivity for antinuclear antibodies, that overlapped with those of patients with chronic active hepatitis. We conclude that asymptomatic patients with unexplained chronic aspartate aminotransferase elevations of a mild to moderate degree frequently have chronic active hepatitis and that many have cirrhosis. Immunoserologic findings compatible with autoimmune hepatitis are commonly present. Steatohepatitis is the most frequent alternative diagnosis, especially in women, and it is not excluded by the presence of antinuclear antibodies. Differentiation of the disorders is possible only by histologic examination.