Preformed lipid peroxidation products present in the feed may contribute to the total reactive oxygen radical load infants have to deal with and may play a role in the pathogenesis of necrotizing enterocolitis and bronchopulmonary dysplasia. In this study, the occurrence of lipid peroxidation in human milk and feeding formulas for preterm babies was evaluated in vitro. Free linoleic acid (18:2) and its hydroperoxide (18:2OOH) were measured by gas chromatography-mass spectrometry and the concentration of 18:2OOH and the 18:2OOH/18:2 ratio were used as indices of peroxidation. In all feeds peroxidation products were present, but the proportion of peroxidized 18:2 was greater in infant formula. Storage of human milk (+4 degrees C for four days) increased lipid peroxidation. Exposure to light during tube feeding increased peroxidation in infant formula but not in human milk. Different procedures for preparation, storage and feeding may decrease the concentration of these potentially toxic peroxidized lipids in human milk and infant formula.