Several combined vaccines have recently been developed, in order to improve the implementation of immunization programmes and increase the coverage for each vaccine. As the response of preterm infants may vary depending on the vaccination schedule and the vaccine product, it should be evaluated specifically as new vaccines become available. In this study we have examined the antibody response to a combined diphtheria, tetanus, acellular pertussis, and hepatitis B vaccine (DTPa-HBV), given as a primary vaccination course at 3, 5 and 11 months of postnatal age, in 34 preterm infants (mean gestational age (GA) = 32.0 weeks) in comparison with 28 term infants. At the end of the primary course, preterm infants had antibody concentrations for pertussis 69 kDa antigen and diphtheria toxoid that were significantly lower than those of term infants; preterm infants with GA < or = 31 weeks had antibody concentrations for pertussis 69 kDa antigen and HBsAg that were significantly lower than those of preterm infants with higher GA; anti-HBs antibody levels correlated with GA. However, the combined DTPa-HBV vaccine elicited seroconversion to all its components in all but two infants, one term and one preterm, after the second dose and a total seroconversion after the third dose. We conclude that preterm infants may be immunized with a combined DTPa-HBV vaccine, starting at the same chronological age, as term infants.