Background: Small preterm infants often develop constipation when fed formula. Hypothesis: Gastrointestinal transit time (GTT) is at least 2 h shorter when protein hydrolysate formula is fed compared with standard formula. Subjects: 15 Preterm infants (gestational age 29(24-32) weeks, birth weight 1241 (660-1900) g, postnatal age 18 (5-54) days) on full enteral feeds (>150 ml/kg/day) were enrolled .Intervention: Randomised allocation to formula with cow's milk protein or protein hydrolysate. Study design: Formula A contained hydrolysed cow's milk protein, formula B native cow's milk protein. The osmolarity was equal. A cross over design was chosen. Adaptive interim analysis was performed when the data of 15 infants had been obtained (two sided Mann-Whitney U test). Results: At the interim analysis median GTT was 12 h shorter with hydrolysate feeding (p=0.0022).Conclusion: Hydrolysed cow's milk protein improved gastrointestinal passage, but randomised trials are required to investigate whether hydrolysate formulas enable a more rapid establishment of full enteral feeding in preterm infants.