Background Bronchopulmonary dysplasia, intraventricular hemorrhage, necrotizing enterocolitis, and retinopathy of prematurity may be different manifestations of oxygen radical diseases of prematurity (ORDP). Objective To test the hypothesis that the antioxidant capacity of cord blood serum will predict risk of ORDP. Design An inception cohort of premature neonates was followed up from birth until discharge or death to determine if outcome was related to cord blood serum antioxidant capacity, as determined by a manual assay measuring the relative inhibition of oxidation of 2,2′-azino-di-(3-ethylbenzthiazoline)-6 sulfonic acid (ABTS). Possible correlations between antioxidant capacity and various perinatal factors were also tested. Setting Level 3 newborn intensive care unit. Patients All inborn very low-birth-weight neonates from whom cord blood was available and for whom maternal consent was obtained were included. Newborns who died in the first week of life or who had major congenital malformations were excluded. A convenience sample of newborns weighing more than 1500 g was used to perfect assay and explore confounders. Main Outcome Measures Significant ORDP was defined as the presence of intraventricular hemorrhage greater than grade 2, retinopathy of prematurity greater than stage 1, bronchopulmonary dysplasia at the postconceptional age of 36 weeks, or necrotizing enterocolitis with the hypothesis that neonates with ORDP will have lower antioxidant capacity in cord blood serum. Results Serum antioxidant capacity at birth correlated with gestational age for the entire sample of 41 neonates and for the 26 neonates born before 32 weeks' gestation. After correction for gestational age, cord serum antioxidant capacity did not correlate with maternal smoking, preeclampsia, chorioamnionitis, cord pH Apgar scores, or any of the ORDP studied. Conclusion Cord serum antioxidant capacity correlates with gestational age but does not predict ORDP risk.