There is some evidence that retinopathy of prematurity is due to excessive oxidative stress on the developing retina caused by high free radical production or reduced ability to eliminate these radicals. To determine the relationship between high levels of oxidative stress and retinopathy of prematurity. A prospective cohort study was designed. Fifty premature infants of less than 33 weeks' gestational age were included. Serum lipoperoxide levels were determined as a measure of oxidative stress. Samples were taken once a week for 1 month, starting from the first week of life. The results of all four samples were compared between infants who developed any degree of retinopathy of prematurity and those without it. Ophthalmological examinations were performed after the fourth week of life. The incidence of retinopathy of prematurity was 22 % (11/50). The mean values of all the samples showed a significant difference between infants who developed retinopathy of prematurity (5.44 +/- 1.30 nmol/ml) and those who did not (2.94 +/- 0.89 nmol/ml, p = 0.0001). The relative risk of developing retinopathy of prematurity with high serum lipoperoxide levels was 5.15, 5.63, 4.15 and 12.70 for each of the weekly samples. There is an association between high serum lipoperoxide levels, as a measure of oxidative stress, and the incidence of retinopathy of prematurity.