BACKGROUND: A recent study proposed a risk factor scoring system for prediction of hyperbilirubinaemia that assigned increased risk to infants of higher birth weight. OBJECTIVE: To investigate this novel finding in a large, retrospective cohort analysis. METHODS: 105 384 newborns (> or =2000 g and > or =36 weeks) were analysed, and the effect of higher birth weight on total serum bilirubin (TSB) > or =342 micromol/l was reported using logistic regression to control for gestational age, scalp injury diagnosis, maternal diabetes, method of delivery and other confounders. RESULTS: The odds ratio for the effect of an additional 500 g of birth weight on TSB > or =342 micromol/l declined with increasing gestational age from 1.55 (95% CI 1.28 to 1.87) at 36 weeks to 1.30 (95% CI 1.12 to 1.50) at 37 weeks and 1.14 (95% CI 1.01 to 1.29) at 38 weeks. There was no association for infants > or =39 weeks. CONCLUSION: Higher birth weight predicts TSB > or =342 micromol/l in 36-38 week infants, but not in infants > or =39 weeks. Further research should explore the causal mechanism for the association in less-mature infants.