New serological tests allow one to classify the majority of patients with acute viral hepatitis into patients with hepatitis B, with hepatitis A or with hepatitis NonA/NonB. This breakdown was performed with 207 consecutive patients from the Zurich area in 1979. The data were compared with those obtained in patient series from 1977/78 (220 patients) and series published earlier (another 790 patients since 1972). Before 1979 only hepatitis B and hepatitis NonB could be differentiated. Since 1972 approximately 50% of all patients have had hepatitis B. In 1979 25% of the remaining patients had hepatitis A and 13% hepatitis NonA/NonB, while the others could not be classified. In all series so far analyzed slightly more men than women were affected, almost half of the patients were between 15 and 29 years old and no seasonal peaks were apparent. Sporadic disease is the rule, and clusters are extremely rare. However, there was a slight shift of risk situations which might have led to transmission of the virus. The percentage of patients with appropriately timed blood transfusions dropped from 12% in 1972 to 5% in 1979 and medical staff members from 18% to 6%, while the percentage of patients with histories of drug use increased from 4% to 11% and of travellers to foreign countries from 15% to 40%. Most of the travellers had hepatitis A, and the majority of transfused patients had hepatitis B, while drug users had either hepatitis B or NonA/NonB. Some background data on hepatitis viruses are summarized and possible prophylactic measures are discussed.