Unbound bilirubin, bilirubin binding capacity, and bilirubin binding affinity were determined by the horseradish peroxidase method at the time of maximum hyperbilirubinemia and/or before exchange transfusions in 13 preterm infants who later died and had autopsies performed. Five of the 13 infants had kernicterus at autopsy. There were no significant differences in weight, gestational age, highest indirect bilirubin level, albumin concentration, severity of acidosis, use of assisted ventilation, sepsis, or other major clinical complications between the five infants with kernicterus and the eight infants without kernicterus. Compared with the eight nonkernicteric infants, the five kernicteric infants had significantly higher unbound bilirubin concentrations (13 ± 10 vs 27 ± 9 nmoles/liter, respectively, P < .05) and significantly lower bilirubin binding capacity and affinity. The data suggest an association between low bilirubin binding capacity and affinity, increased unbound bilirubin, and kernicterus in preterm infants with severe clinical complications.