A follow-up serological study was conducted involving 47 subjects who received four doses of diphtheria and tetanus toxoids combined with acellular pertussis vaccine (DTaP) together with Salk-type inactivated polio vaccine (DTaP-wIPV) until 6 years of age. All antibody levels declined more rapidly than expected within 3 years after the completion of the primary vaccination with 4th booster dose and titers persisted until 6 years of age. The positive rate of the IgG antibody against pertussis toxin (PT) was 31.9% (15/47) at 4 years of age, 41.0% (16/39) at 5 years of age, and 40.5% (15/37) at 6 years of age. A significant increase in the PT antibody was observed in 6 subjects, suggesting subclinical infection. Positive rates of antibodies against other targets did not decrease but titers of neutralizing antibodies against poliovirus type III decreased in only a few subjects. These data suggest the need for an additional preschool booster immunization using DTaP-wIPV.