Lipid peroxidation was measured in 35 preterm infants by determining serum malondialdehyde (MDA) levels during the fist week of life. Serum concentrations of MDA were measured at one hour, 24 hours, 48 hours and one week of age. There were no correlations between gestational age, birth weight, and serum malondialdehyde levels at one, 24, or 48 hours, or on day seven. Serum MDA levels were higher in infants requiring mechanical ventilation compared to those breathing spontaneously, but the difference was not statistically significant. There were no correlations between MDA levels and corresponding fraction of inspired oxygen (FiO2) levels and arterial oxygen tension. No significant difference in serum MDA concentration was seen in relation to intravenous feeding with lipid emulsions. Serum MDA concentrations of infants with sepsis were not different at hours one, 24 and 48, but were significantly higher on day seven, when compared with infants without sepsis. MDA levels in the first and second serum samples were significantly higher in infants who were born after spontaneous vaginal delivery, compared to those born by cesarean section.