The measurement of malondialdehyde (MDA) in biological fluids remains a popular method for the quantification of free radical damage to lipids in vivo. Several diseases of prematurity are thought to be related to oxidative injury and previous studies have found elevated MDA in plasma and urine in preterm infants. Our aim was to investigate the relationship between plasma and urinary MDA levels in preterm infants during the first week of life using a high-performance liquid chromatography (HPLC) based, thiobarbituric acid (TBA) assay with paired plasma and urine samples. We obtained 50 paired samples, and were unable to demonstrate a relationship between the two parameters after the first day of life. In 18 cases a further urine sample was collected 24 h later. There was a positive correlation (r = 0.54, P = 0.02) between plasma MDA and urinary MDA 24 h later. The finding that plasma changes in MDA are reflected in urine 24 h later validates the use of urinary MDA as a marker of whole body lipid peroxidation in populations without renal disease.