The etiology and epidemiology of 547 consecutive episodes of acute viral hepatitis in adults and diagnosed in a general hospital over 12 years (1983-1994) were prospectively analyzed as were the changes observed during the two halves of the study period. Of the 547 episodes, 25.4% were of type A, 41.1% type B, 21.9% type C, 6.6% non A, non B, non C, 2.4% type D, 1.1% by cytomegalovirus and 1.4% by the Epstein-Barr virus. The proportion of hepatitis A increased from 21.5% from 1983-1988 to 34.1% from 1989-1994 (p = 0.002), while hepatitis C decreased from 24.9% to 15.3% (p = 0.01) during the same periods. The proportion of hepatitis B observed in intravenous drug addicts fell from 56.1% in the first period to 39.3% in the second period (p = 0.03), while sexually transmitted hepatitis B rose from 7.3% to 22.9% (p = 0.002). A decrease was observed in the cases of hepatitis C in both periods in the intravenous drug addict cases (60.6% vs. 34.6%; p = 0.03) with an increase being observed in the C virus transmitted by unapparent mechanisms (2.1% vs. 23.1%; p = 0.001). These results suggest that modifications may currently be observed in the epidemiology of the viral hepatitis in Spain and that these trends should be taken into account when planning preventive strategies.