The major features of the epidemiology of hepatitis A which are revealed by an analysis of morbidity data are a cyclic pattern with peaks of incidence every 5 to 10 years. An increase in the number of cases at certain times of the year; a peak incidence among young children; a high secondary attack rate among family members, and a low case-fatality rate. The age distribution of cases of hepatitis A would be expected to follow the pattern of other enteric viral infections such as poliomyelitis and to be affected by levels of hygiene and sanitation. Certainly the pattern of transmission of HAV in the community is markedly different from the pattern produced by respiratory viruses. Data on the length of infectivity of patients with hepatitis A are based largely on studies in human volunteers supplemented by epidemiological observations and, to a limited extent, by studies in experimentally infected animals and in cell culture.