Lipid peroxidation was measured in 19 very low birth weight infants with respiratory distress syndrome by quantitating ethane and pentane in expired air during the first 5 days postnatally. Despite high levels of inspiratory oxygen, the ethane and pentane output was low within the first 24 hours; thereafter it increased up to 100 and 30 fold, respectively. On days 1 to 3 there was no detectable correlation between lipid peroxidation and fractional inspiratory oxygen. However, on days 4 and 5, lipid peroxidation and fractional inspiratory oxygen showed a significant correlation. Maximal amounts of expired ethane and pentane were significantly higher for patients with a poor outcome (five deaths, six cases of bronchopulmonary dysplasia) than for those with good outcome (eight infants surviving intact) (p less than 0.01). The results imply a role for free oxygen radicals in the pathogenesis of life-threatening complications in the very low birth weight infant.