One hundred twenty-two hepatitis B surface antigen (HBsAg) carrier infants were followed-up for 8-10 years. One hundred eleven had antibody to hepatitis B core antigen (anti-HBc; 83 had been vaccinated) and the remaining 11 were without anti-HBc (7 had been vaccinated). During the follow-up period, 29 (26.1%) carrier infants with anti-HBc had one or more episodes of alanine aminotransferase (ALT) elevation and up to 32.8% (21/64) of the carriers in this group lost their hepatitis B e antigen (HBeAg) before the age of 10. In addition, 2 (1.8%) carriers lost their HBsAg at the age of 3 and 8, respectively. No significant symptom or sign was noted during HBeAg seroconversion. In contrast, all the carrier infants without anti-HBc were still positive for both HBeAg and hepatitis B virus (HBV) DNA and none displayed abnormal ALT levels or any symptom related to liver disease. One became anti-HBc positive at the age of 9, and 5 other carriers had inconsistent borderline or weakly positive titers of anti-HBc. The episodes of ALT elevation and the prevalence of HBeAg seroconversion were not significantly different between immunized carrier infants. In conclusion, HBeAg seroconversion may occur in about one third of the anti-HBc-positive carrier infants during the first decade. On the other hand, the anti-HBc-negative HBsAg carrier infants' immune incompetence to the HBV antigens could persist for more than 10 years. Hepatitis B immunization did not have significant effect on the clinical course in carriers.