In a retrospective study, urinary malondialdehyde concentration in 45 preterm neonates (25-35 weeks' gestation) during their first month of life was measured by HPLC. Urine was collected on different days of life as a 3-h sample. The frequency of urine collection and measurement varied between one (n = 22) and seven times (n = 8) per child. The study group was divided into three categories according to birth weight: low-birth-weight infants (LBW) (n = 16), very low-birth-weight infants (VLBW) (n = 17) and extremely low-birth-weight infants (ELBW) (n = 12). Urinary malondialdehyde concentration was highest in the ELBW group: 1.15 (0.66, 2.12) mumol/l (median and quartiles) versus 0.58 (0.34, 1.18) mumol/l in the VLBW and 0.60 (0.40, 1.06) mumol/l in the LBW groups (ELBW versus VLBW, p < 0.005; ELBW versus LBW, p < 0.02). In oxygen-treated neonates, significantly higher malondialdehyde values were found compared to those without supplementary oxygen (0.89 (0.48, 1.74) versus 0.58 (0.32, 0.89) mumol/l; p < 0.005). Likewise, a higher malondialdehyde concentration was found in infants requiring mechanical ventilation (intermittent mandatory IMV or high frequency ventilation) compared to those breathing spontaneously (intermittent mandatory ventilation: 0.80 (0.42, 1.66); p > 0.05 and high frequency ventilation: 1.20 (0.83, 2.13); p < 0.001 versus 0.57 (0.33, 0.88) mumol/l). Malondialdehyde concentrations correlated significantly with FiO2 values of the individual patients (r = 0.22; p < 0.02). Comparing urinary malondialdehyde concentrations in infants with and without bronchopulmonary dysplasia, a significantly higher malondialdehyde concentration was found in the former group (0.96 (0.51, 2.07) versus 0.60 (0.32, 0.98) mumol/l; p < 0.005)).(ABSTRACT TRUNCATED AT 250 WORDS)