The combined hexavalent diphtheria-tetanus-acellular pertussis-hepatitis B-inactivated polio Haemophilus influenzae type b (DTPa-HBV-IPV/Hib) vaccine produces similar hepatitis B responses as the HBV monovalent vaccine. Booster vaccination of immunocompetent individuals primed against hepatitis B in infancy is currently not recommended. We investigated persisting immunity to hepatitis B in 4-6 (Study A; 106745) and 7-9 (Study B; 106744) year-old children primed in infancy and boosted in the second year of life with DTPa-HBV-IPV/Hib. Immunity was assessed by measuring persisting anti-HBs antibodies and evaluating the response to a challenge dose of HBV vaccine. At 4-6 years of age 86.0% of 186 subjects had persisting anti-HBs > or =10 mIU/ml increasing to 98.4% after the challenge. At 7-9 years of age, 78.0% of 186 subjects continued to have anti-HBs antibody concentrations > or =10 mIU/ml, increasing to 98.9% after the challenge. In both studies anti-HBs antibody GMC rose >80-fold. An anamnestic response to the HBV challenge was observed in 95.7% and 98.9% of subjects in Studies A and B, respectively. In both studies, 87% of 38 subjects with initially undetectable circulating anti-HBs antibodies (>3.3 IU/ml) achieved the 10 mIU/ml threshold after challenge; > or =97.0% of subjects with detectable antibodies before the challenge at least quadrupled their concentration. Post-vaccination anti-HBs concentrations were directly related to persisting antibody concentrations and the concentrations achieved after the booster dose in the second year of life. The HBV vaccine challenge dose was well tolerated. These studies show that primary and booster vaccination with combined DTPa-HBV-IPV/Hib (Infanrix hexa) induces sustained immune memory against hepatitis B up to age 9 years.