The purpose of this study was to determine whether protein carbonyls and the lipid peroxidation product malondialdehyde (MDA) are elevated in plasma from very low birth weight (<1500 g) infants, whether they are affected by selenium supplementation, and whether they are associated with poor respiratory outcome or retinopathy. The study group comprised 173 infants enrolled in a randomized controlled trial of selenium supplementation. Plasma samples, collected before randomization, at 7 and 28 d after birth, and at 36 wk postmenstrual age, were analyzed for protein carbonyls and total MDA. Respiratory outcome was assessed as oxygen requirement at 28 d of age or 36 wk postmenstrual age and as number of days on oxygen. Protein carbonyl concentrations in very low birth weight infants were significantly higher than for adults but lower than for cord blood from term infants. Median values decreased significantly by 28 d, and there was no relationship with birth weight. MDA concentrations in very low birth weight infants overlapped the ranges for healthy adults and cord blood from term infants. They correlated positively with birth weight at 28 d but not at other times. Supplementation almost doubled plasma selenium concentrations, but carbonyls and MDA did not differ between the supplemented and unsupplemented groups. There were no significant differences in oxidant marker levels in infants who did or did not develop chronic lung disease or retinopathy. Protein carbonyls and MDA measurements in plasma do not show evidence of systemic oxidative stress in <1500-g infants and are not affected by selenium supplementation. Oxidative injury at sites such as the lung may be important in prematurity, but markers from such sites must be measured to relate to outcome and antioxidant supplementation.