From 1971 to 1975, HBV-induced hepatitis was observed in 80 children. The diagnosis was based upon the detection in serum of HBsAg and/or the secondary occurrence of anti-HBs. Thirty-one patients presented with acute viral hepatitis, 16 with severe or fulminant hepatitis, 17 with chronic persistent hepatitis, 12 with chronic active hepatitis, and 4 were asymptomatic chronic carriers of HBsAg. Twenty-nine of 80 children were under one year of age (36%), the peak of frequency occurring from 2 to 5 months. The source of infection, determined in 27 of 29 infants, was administration of blood derivatives in 15 cases and contact with an HBsAg carrier mother in nine instances. In the latter type, the incubation time (103 days) was compartible with an oral route of infection, Persistent antigenemia occurred in only 3 of 29 patients. The overt type of disease developed by most infants, as well as the small number of patients who became HBsAg carriers, suggest that the carrier state, often encountered in neonatally infected infants in other countries, may be related to environmental or genetic factors rather than to immaturity of theimmune system.