Neurologic and developmental performance during the first year of life was correlated with maximum neonatal serum bilirubin levels for 27,000 infants in the Collaborative Perinatal Project. The infants were grouped by race and by five birth weight/gestational age categories to control for the effect of these factors on hyperbilirubinemia and developmental outcome. Low mean eight-month motor scores and delayed one-year motor development were associated with serum bilirubin levels in the range of 10 to 14 mg/dl and above. This relationship was strongest for low-birth-weight/short-gestational-period infants. A persistent association of developmental outcome with hyperbilirubinemia was found over and above the variation of maturity within the birth weight/gestational age categories.