In a cohort of 829 preterm infants (birth weight below 1500 g) we identified 13 monozygotic, 10 same-sex dizygotic, and 12 same-sex matched singleton pairs. The difference in daily weight gain within pairs was significantly lower in monozygotic twins compared with dizygotic twins or matched singleton pairs. Our data support a strong genetic influence on postnatal growth in preterm infants. Therefore, weight gain of preterm infants may be an interesting model to study polymorphic variants of genes regulating neonatal resorption, metabolism, or energy expenditure, and their influence on weight gain in preterm infants.