OBJECTIVE To determine and compare the etiology of sporadic acute and fulminant viral hepatitis in two groups of patients 16 years apart. METHODS Serologic diagnostic tests for hepatitis A, B, C, D and E, and cytomegalovirus infection were carried out in 276 patients during 1994-1997 (Group A) and 206 patients during 1978-1981 (Group B). RESULTS Among children, hepatitis A virus was the major etiologic agent (81.6% in Group A and 51.4% in Group B), followed by hepatitis E virus (12.2%, 46.4%) and hepatitis B virus (5.4%, none). Among adults, hepatitis E virus was the main causative agent (42.4% in Group A and 71.2% in Group B) followed by HBV (28%, 25.5%) and hepatitis A virus (10.6%, 3.5%). Delta hepatitis was found only in Group A. No viral cause was found in 25% of patients in Group A and 13.5% patients in Group B. CONCLUSIONS Hepatitis E virus is a major cause of sporadic acute and fulminant hepatitis. There has been an increase in hepatitis A in adults who developed fulminant hepatic failure. Our data points to the emergence of hepatitis A in adults and emergence of delta virus infection. Hepatitis C virus was unimportant in causing sporadic hepatitis.