To evaluate the tuberculin response after bacille Calmette-Guérin (BCG) vaccination in preterm infants. Thirty-five infants born at <35 weeks gestation were included in the study. An intradermal injection of 0.05 ml BCG vaccine was given to each infant at postnatal months 2-3. Tuberculin skin tests (TSTs) were done 8-12 weeks after vaccination using 0.1 ml of 5 TU purified protein derivative (PPD). The diameter of induration was measured 72 h later and > or =5 mm induration was taken as a positive response. The babies had a mean birth weight (SD) of 1650 (424) g, and a gestational age of 32.4 (2.1) weeks. The TST was positive in 20 (57%) of the infants. The mean body weight of the tuberculin-positive babies was significantly higher than the others at both vaccination and TST. No difference was found between groups concerning sex, birth weight, gestational age, intrauterine growth and postnatal age at BCG vaccination and TST. BCG vaccination in preterm infants at months 2-3 of postnatal life results in a high percentage of BCG scarring and 57% TST conversion. A positive tuberculin response was significantly related to the postnatal weight gain of the preterm infants.