A randomized controlled trial was done to compare seroconversion following a single dose of trivalent oral polio vaccine (TOPV) in preterm babies, vaccinated at two different post-conception ages, with that of term newborns. Sixty-two consecutive preterm babies < or = 35 weeks were randomly allocated to two groups. Group A was vaccinated 'early' at 34-35 weeks and group B 'late' at 38-40 weeks post conception. The two groups were comparable in birthweight [mean (SD) 1594 g (118) and 1599 g (126), respectively] and gestational age [mean (SD) 33.2 (1.2) and 33 (1.3) weeks, respectively]. A control group of 36 term babies (group C) were vaccinated in the 1st week of life. Polio virus antibodies were measured immediately before and 6-8 weeks after vaccination. Group A had seroconversion rates of 54.2, 12.5 and 12.5% against polio virus types 1, 2 and 3, respectively, group B had rates of 60.0, 8.0 and 16.0%, and group C rates of 53.6, 10.7 and 14.3%. Differences in the seroconversion rates in the three groups were not statistically significant. The conversion rates against types 2 and 3 are much lower than in previous studies. We conclude that preterm babies vaccinated at 34-35 weeks post conception show seroconversion rates similar to those in term newborns.