Between January 1980 and December 1983, 332 consecutive cases of acute hepatitis were observed in adult patients admitted to the Department of Infectious Disease and Gastroenterology of Siena. Sex and age of the patients, the presence of jaundice, the maximum value of the serum-glutamate-pyruvate-transaminase (SGPT) were considered. Serum specimens were tested for hepatitis B surface antigen (HBsAg), antibody against hepatitis B core antigen (anti-HBc) of the IgM class, antibody against hepatitis A virus (anti-HAV) of the IgM class, antibody against cytomegalovirus (CMV) of the IgM class, Paul Bunnel Davidshon reaction. Hepatitis A was diagnosed in 25 cases (7.5%). Hepatitis B in 167 (50.3%). Hepatitis due to CMV in 2 cases (0.6%). And, by exclusion, hepatitis non A, non B in 138 cases (41.6%). Male patients were affected with significantly higher frequency than female (p less than 0.01); the same was seen for young patients (14-30 years) compared to the older ones (31-50 years, and over 50 years) (p less than 0.01 in both). Biochemical investigation showed that hepatitis A and B had a significantly higher, maximum SGPT value than hepatitis non A non B (p less than 0.01 in both). Icteric patients were significantly more frequently observed among hepatitis A and B cases than hepatitis non A non B cases (p less than 0.01 in both).