The etiology of sporadic acute hepatitis was studied in 334 consecutive patients from Taiwan (237 men and 97 women, aged 16-81 years), with emphasis on the role of hepatitis C virus (HCV), hepatitis E virus (HEV), and GB virus-C/hepatitis G virus (GBV-C/HGV) in acute non-A, non-B (NANB) hepatitis and in HBsAg carriers with superimposed acute hepatitis. According to the conventional diagnostic criteria, there were 12 cases (3.6%) of acute hepatitis A, 17 cases (5.1%) of acute hepatitis B, 128 cases (38.3%) of acute NANB hepatitis, and 177 cases (53.0%) of acute hepatitis in HBsAg carriers (those who were HBsAg positive but IgM anti-HBc negative). Among 128 cases of acute NANB hepatitis, 70 (54.7%) had acute hepatitis C (HCV RNA positive), 5 (3.9%) had acute hepatitis E (IgM anti-HEV positive), and the other 53 (41.4%) were presumably acute hepatitis non-A-E. The prevalence of acute hepatitis A, B, E, and non-A-E showed no significant sex difference, whereas acute hepatitis C was significantly more prevalent in females. The prevalence of acute hepatitis A and B decreased and that of acute hepatitis C increased significantly with increasing age. In contrast, acute hepatitis E and non-A-E showed no significant age predominance. Of 177 HBsAg carriers with acute hepatitis, 64 (36.1%) demonstrated non-B hepatotropic virus superinfection, with HCV being the most common (60.9%), followed by hepatitis D, E, and A viruses, and the other 55 (31.1%) and 58 (32.8%) were presumed to have acute exacerbation of chronic hepatitis B or superimposed acute hepatitis non-A-E, respectively. Serum GBV-C/HGV RNA was detected in 3-4% of acute hepatitis non-A-E cases, suggesting its limited role in these cases.