Some studies have suggested that decreased seroconversion rates might be found in premature infants with low birthweight (< 2000g) following administration of hepatitis B vaccine at birth. The aim of the present investigation was to evaluate possible differences in seropositive rates between full-term and preterm infants after primary vaccination, in particular when gestational age or birthweight is very low. Two-thousand and nine neonates born to HBsAg-negative mothers were vaccinated with 10 microg of recombinant hepatitis B virus (HBV) vaccine, from May 1991 to October 1994. Children with infections, congenital malformations or serious illnesses were excluded. HBV vaccine was administered intramuscularly, on the fourth day of life and again at 1 and 6 months of age. A 1-ml blood sample was drawn from each infant 1 month after the third vaccine dose for determination of the level of anti-HBs antibody. The response to HBV vaccination was evaluated in 241 preterm (gestational age <38 weeks) infants and 1727 term neonates. No statistical difference was observed in the distribution of anti-HBs antibody level, either between preterm infants (<38 weeks) and newborns of normal gestational age, or between low birthweight (<2500 g) and normal weight infants. The results suggest that preterm and low birthweight infants (<2500g) respond to HBV vaccine in the same measure as normal-term infants.