AIM To determine lipid peroxide concentrations in the first 10 days of life. METHODS Malondialdehyde concentrations were investigated in neonates with or without hyperbilirubinaemia during the first 10 days of life. RESULTS Serum malondialdehyde concentrations were higher in infants with hyperbilirubinaemia than in controls. A positive correlation was found between malondialdehyde and bilirubin concentrations in the study group. When the study group was categorised according to the presence of haemolysis, a significant correlation was found between malondialdehyde and bilirubin concentrations in those infants with hyperbilirubinaemia due to haemolysis. There was no such correlation in those without haemolysis. CONCLUSION Exchange transfusion rapidly produces variable changes in pro-oxidant and antioxidant plasma concentrations in neonates, which may be responsible for free radical metabolism. The fall in malondialdehyde concentration is probably directly related to its exogenous removal by exchange transfusion.