The authors investigated families with at least one hepatitis B surface antigen (HBsAg) carried in Okinawa, Japan, to determine possible routes of hepatitis B virus transmission within family units. A total of 175 members of 37 families on the western part of Iriomote Island, Okinawa, were followed up for 2-13 years; 68 were HBsAg-positive at least once. Sera were collected once a year. All serum samples were assayed by radioimmunoassay to determine presence of HBsAg, antibody to HBsAg (anti-HBs), hepatitis B e antigen (HBeAg), antibody to hepatitis B e antigen (anti-HBe) and antibody to hepatitis B core antigen (anti-HBc). In five of the 15 families with a positive mother and negative father, one or more HBsAg-positive children were found. In contrast, an HBsAg-positive child was found in only one of the eight families with a negative mother and positive father. In nine of the 14 families with both parents negative, two or more children were positive. Apparent sibling-to-sibling hepatitis B transmission occurred during the period of observation in four of the families with both parents negative. Eight children less than four years old were HBsAg-positive when first surveyed or became HBsAg carriers when four years of age or younger. No one over four years of age changed from HBsAg-negative to positive. This study found 1) in the family setting, there were cases of apparent maternal transmission but there were more cases of apparent sibling-sibling transmission, and 2) children under four years of age seemed to become carriers more easily than older children.