The clinical, virologic and pathologic features of chronic hepatitis B virus infection were studied in 66 children, of whom 29 were symptomatic and 37 asymptomatic. The majority (79%) of symptomatic children had histologically aggressive diseases: 11 had chronic active hepatitis and 10 had cirrhosis. In contrast, most asymptomatic children had nonaggressive diseases (35 cases); only 2 had chronic active hepatitis. Nine of the 10 children with cirrhosis were under 6 years of age, and the cirrhosis was often advanced, indicating that hepatitis B virus infection can cause the rapid development of cirrhosis in early life. HBcAg was present in 71% of 62 cases examined and correlated well with the status of HBeAg in serum. Cytoplasmic HBcAg was more frequently associated with aggressive disease than was nuclear HBcAg expression alone or no detectable HBcAg in the liver. A male predominance (75%) was found, particularly in children with aggressive diseases (91%) compared to those with nonaggressive forms of disease (67%). Sera from mothers of 43 of these children were tested for HBsAg, and 51% were positive. HBsAg was particularly common among mothers of children with symptomatic disease (69%) or cirrhosis (100%). These findings suggest that male sex and perinatal infection are important factors in the development of overt chronic hepatitis B and cirrhosis in children.