Viral etiology was investigated in 133 Saudi patients with acute hepatitis seen in King Khalid University Hospital, Riyadh. between July 1993-May 1994. Out of the 133 patients, 51 (38.3%) were diagnosed as having acute hepatitis due to hepatitis A virus (HAV). Hepatitis C virus (HCV) was the second most common etiological agent (20.3%). There were 35 patients with acute hepatitis (26.3%) in whom no viral marker for HAV, HBV, HCV, CMV or Epstein-Barr virus (EBV) was detected. Among the 51 patients with acute hepatitis due to HAV, the majority (88.2%) were children (1-12 years) and only 6 (11.8%) were adults (15-24 years). This is in contrast to patients with HCV or HBV infection where the majority were adults: 74.1% and 82.3% respectively. The diagnosis of acute hepatitis due to HAV in Saudi adults, an observation not seen earlier, indicated a change in the epidemiological pattern of HAV infection in the Saudi population. This change was confirmed by the significant reduction in the prevalence of anti-HAV in 630 Saudi subjects (1-30 years old) (50.2%) investigated in 1994 compared to that of 587 subjects of the same age group investigated in 1986 (76.5%) (P < 0.005). In the light of these results, a nation-wide survey is recommended to confirm this pattern in other areas. It is important that high-risk Saudi groups be identified and evaluated for their anti-HAV status as these groups are candidates for HAV vaccination.