Feeding intolerance (FI) is usually defined as "gastric residual volume of more than 50 % of the previous feeding volume, emesis, abdominal distension or both of these symptoms and a decrease, delay or discontinuation of enteral feedings." We aimed to compare the incidence of FI in preterm infants fed with powdered or liquid infant formula, and in a prospective, double-blind, pilot study, 78 preterm infants were randomized to receive powdered or liquid form of the same preterm infant formula. The primary outcomes were the incidence of FI in both groups. The pH of gastric fluids was measured in the fasting and postprandial periods on the seventh day of life, and gastrointestinal complications were recorded during the hospitalization period. The incidence of FI was significantly higher in infants fed with liquid formula (n = 34) when compared with infants fed with powdered formula (n = 44) [9 (26.5 %) vs 2 (4.5 %), p < 0.01, respectively]. The median fasting gastric fluid pH was significantly lower and postprandial gastric fluid pH was significantly higher than in infants fed with powdered formula (2.9 vs 3.4, p < 0.01 and 6.0 vs 5.9, p < 0.05 respectively). Infants fed with liquid formula regained birth weight significantly later than infants fed with powdered formula (9.5 vs 8.0 days, p < 0.01). Conclusion: Although the exact mechanisms are not clear, increased incidence of FI and delayed growth in the first weeks of life in preterm infants fed with liquid formula might be caused by altered gastric acidity or possible disrupted protein bioavailability due to different production and sterilization processes.