AIM: To assess the variation in size of the thymus in vivo in preterm neonates and to identify relations between thymic size and gestational age (GA), birthweight, occurrence of postnatal infections and maternal alcohol and tobacco intake during pregnancy. METHODS: Eighty preterm neonates with a GA between 24 and 36 wk, and a birthweight between 490 and 4110 g were examined between days 0 and 19 after birth. The thymic size was assessed by sonography as a volume estimate, the so-called thymic index (Ti). The median Ti was 5.2 (1.2-17.9). Ti was positively correlated with birthweight and GA and negatively correlated with occurrence of postnatal infection (p < 0.01, p = 0.03, p = 0.05, R2 = 0.68). A correlation between thymic size and maternal alcohol and tobacco intake was not demonstrated. CONCLUSION: It is possible to assess the size of the thymus by sonography in very low-birthweight and preterm neonates. A normal range for Ti in preterm neonates has been established. The sonographic method is a safe and effective technique for measuring the size of the thymus in preterm infants.