Inhaled nitric oxide (iNO) improves oxygenation in premature infants, but conc ern has been raised about its potential oxidative toxicity. We designed this stu dy to assess the oxidative balance in premature infants who were exposed to low dose iNO and the relationship with their clinical outcome on day 28 of life. Ato tal of 274 infants who were < 32 wk gestation were randomized at birth to receiv e 5 ppm of iNO if they presented with hypoxemic respiratory failure. Nonhypoxemi c infants were studied as the reference group. Blood samples were withdrawn 24 h apart, within the first 4 d of life, to assess malondialdehyde (MDA)-concentra tion as oxidative stress marker and total plasmatic glutathione (GSH), intraeryt hrocyte GSH peroxidase, and GSH reductase activities as antioxidant defenses. Af ter 24 h, the rise in MDA was blunted in the iNO group compared with controls an d was close to the reference infants. Conversely, GSH was more stable in the iNO group, when there was no difference for the GSH peroxidase and GSH reductase ac tivities. On day 28, Oxygen dependence was linked with a higher increase in MDA as was the risk for death, whereas intraventricular hemorrhage was associated wi th a higher initial drop in GSH. Early low-dose iNO in hypoxemic preterm infant s improves oxidative balance and seems to be clinically beneficial up to day 28 of life. Improve the oxygenation in premature infants, but conc ern has been raised about its potential oxidative toxicity. We designed this stu dy to assess the oxidative balance in premature infants who were exposed to low dose iNO and the relationship with their clinical outcome on day 28 of life. Ato tal of 274 infants who were <32 wk gestation were randomized at birth to receiv e 5 ppm of iNO if they presented with hypoxemic respiratory failure. Nonhypoxemi c infants were studied as the reference group. Blood samples were withdrawn 24 h apart, within the first 4 d of life, to assess malondialdehyde (MDA) -concentra tion as oxidative stress marker and total plasmatic glutathione (GSH), intraeryt hrocyte GSH peroxidase, and GSH reductase activities as antioxidant defenses. h, the rise in MDA was blunted in the iNO group compared with controls an d was close to the reference infants. Conversely, GSH was more stable in the iNO group, when there was no differen ce for the GSH peroxidase and GSH reductase ac tivities. On day 28, Oxygen dependence was linked with a higher increase in MDA as was the risk for death, anti intraventricular hemorrhage was associated wi th a higher initial drop in GSH. Early low-dose iNO in hypoxemic preterm infant s improves oxidative balance and seems to be clinically beneficial up to day 28 of life.