In a 4-month period, 216 cases of acute viral hepatitis were diagnosed in adults at the Infectious Diseases Hospital, Athens, Greece. Twenty-six percent of these were hepatitis B surface antigen negative. A full set of clinical specimens was obtained from 19 of these patients, who were studied in depth for the etiology of their hepatitis. A total of 7 of the 19 patients had serological evidence of hepatitis A virus infection, and 2 had evidence of recent hepatitis B virus infection. The remaining 10 patients lacked evidence of hepatitis A virus, hepatitis B virus, cytomegalovirus, or Epstein-Barr virus infection related to their illness and were classified as having type non-A, non-B hepatitis. Types A and non-A, non-B hepatitis were clinically similar in these adults patients. However, patients with type non-a, non-B hepatitis were, on the average, older and more likely to have received parenteral inoculations during the 6 months before contracting hepatitis. Approximately 76% of the 216 consecutive cases of acute viral hepatitis were probably type B, approximately 10% were probably type A, and approximately 14% were probably type non-A, non-B, although the latter two types may be underestimated because of the possibility of superinfection with these viruses in asymptomatic hepatitis B surface antigen carriers. Thus, all three types of viral hepatitis appear to be important in the etiology of liver disease in Athens, Greece.