Epidemiological aspects of 400 consecutive in-patients with acute viral hepatitis, aged one to 81 years, were studied: 180 patients were less than 15 years old (mean age 6.9 years) and 220 were older (mean age 33.8 years). Serum hepatitis markers were detected by RIA. Hepatitis A (HA) was diagnosed in 188 patients (47.0%), ages ranging from one to 34 years (mean age 8.5 years), hepatitis B (HB) in 163 patients (40.8%), ages ranging from two to 72 years (mean age 32.7 years) and non-A, non-B hepatitis (NANBH) in 41 patients (10.2%), ages ranging from two to 81 years (mean age 35.4 years). Two patients had simultaneous HA and HB, and in six patients the etiology was not classified. In patients less than 15 years old, HA was the commonest type (91.1%), other etiologies having a low frequency; in the age group over 15 years, HB represented the most frequent type (70.0%), followed by NANBH (16.8%) and HA (10.9%). The probable source of infection was unknown in most of the cases (51%). With regard to HA, secondary contact was frequently reported and small outbreaks were registered. Parenteral exposure was the main probable source of HB and NANBH virus infections, the transmission frequently being iatrogenic. Drug abuse was a rare source of infection.