Recent advances in the study of hepatitis B shed much light on the basic epidemiologic patterns of this unique infection. The spectrum of host responses following exposure is unusually wide. Hepatitis B appears to be one of the most widespread infections; the total number of chronic carriers of the hepatitis B antigen has been estimated as at least 120 million. The following factors have been found to be closely associated with the risk of hepatitis B; geography, sex, age at testing, age at primary infection, socioeconomic status, ethnicity, occupation, sharing a household with a carrier, sexual promiscuity, and immunologic responsiveness. Data concerning the infectivity of an asymptomatic carrier are ambiguous; whether a carrier can transmit the virus by sexual intercourse also remains unknown. Available evidence seems to suggest that genetic factors may be of importance in the aggregation and segregation of hepatitis B.