One hundred and sixteen Korean adults with biopsy-proven acute viral hepatitis were studied to determine the etiology and the outcome of the disease using paired sera obtained during acute and convalescent phases. The prevalence of acute viral hepatitis A, B, D and non-A non-B were 3.4%, 60.3%, 0.9% and 35.3%, respectively: hepatitis B virus infection was the most common cause and the hepatitis D virus superinfection was almost negligible. Only eleven (26.8%) of 41 patients with AVH NANB were negative for all serological markers of HBV. The rest (73.2%) were positive for at least one HBV marker: HBsAg was positive in 31.7%. Therefore, the presence of HBV serologic markers in the sera does not exclude the diagnosis of AVH NANB in Korea. In patients with acute viral hepatitis B, 27% remained positive for HBsAg. Chronic hepatitis developed in 12.8% and 17% patients with acute hepatitis B and non-A non-B, respectively. Progression to chronic hepatitis in patients with acute viral hepatitis B and non-A non-B occurred more commonly, although statistically not significant, in male sex and in patients who did not have clinical jaundice during the acute phase and who showed bridging necrosis in their liver biopsies. Age did not influence the progression to chronic hepatitis.