Ischemia-reperfusion injury may affect morbidity and mortality in preterm and asphyxiated term infants. Reoxygenation of hypoxic tissues leads to the formation of free oxygen radicals by xanthine oxidase that may induce lipid peroxidation, enzyme inhibition, and DNA strand breakage. We measured arterial cord blood samples from 36 healthy term infants for baseline values and arterial blood sampled at 1 and 4 h after birth from 45 preterm infants admitted for intensive care for serial estimates of plasma xanthine oxidase activity and lipid hydroperoxide levels. Mean +/- SEM plasma xanthine oxidase activity in cord blood of term infants was 2.3 +/- 0.4 mU/mL and lipid hydroperoxide levels were 2.6 +/- 0.3 nmol/mL. Eighteen of the 45 preterm infants met the criteria defining poor outcome (poor outcome group) and had lower umbilical arterial pH and base excess than the 27 preterm infants in the control group. Mean plasma xanthine oxidase activity increased from 2.7 +/- 0.4 at 1 h to 4.7 +/- 0.6 mU/mL at 4 h of age (p < 0.001) in the poor outcome group and decreased from 2.1 +/- 0.3 to 1.1 +/- 0.2 mU/mL (p = 0.004) in the control group. Lipid hydroperoxide levels in the poor outcome group increased from 2.8 +/- 0.6 nmol/mL at 1 h to 4.3 +/- 0.6 nmol/mL at 4 h of age (p < 0.001) and decreased from 2.1 +/- 0.6 to 1.6 +/- 0.2 nmol/mL (p = 0.008) in the control group. At 4 h of age, xanthine oxidase activity and lipid hydroperoxide levels were significantly higher in the poor outcome group than in the controls (p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)