From a national cohort of Danish children, infants 35 weeks or greater gestation with a serum bilirubin of 25 mg/dl or higher in the first three weeks of life were studied, and compared with a matched control group. Infants with intermediate or advanced acute bilirubin encephalopathy were excluded, as were those with Rh isoimmunization and conjugated hyperbilirubinemia. Around 50% of the infants with hyperbilirubinemia had mild (early) signs of acute bilirubin encephalopathy. The median of the peak serum bilirubin level was 26 mg/dl (range 25 – 35). The parents of these infants were asked to complete an Ages and Stages Questionnaire (ASQ) that evaluated the child’s development between 1 – 5 years of age. There were just over 200 infants in each group and the response rates were 70% in the control group and almost 80% in the exposed group. A matched analysis of around 100 pairs and an unmatched regression analysis were both performed. Neither analysis found any significant differences between the two groups in this population-based study. There was no evidence of increased risk of developmental delay in the infants with hyperbilirubinemia compared to controls.