Objective: To determine the unbound bilirubin concentration (UBC) associated with kernicterus with the use of clinical data from clusters of kernicterus after sulfisoxazole and benzyl alcohol administration. Design: Sulfisoxazole at 12 mg/dL and benzoate at 10 mmol/L are associated with kernicterus at total bilirubins near 12 and 10 mg/dL, respectively. The concurrent UBC was estimated by first measuring the drug-induced increases in UBC in plasma and artificial sera (peroxidase-diazo method). The increases were then applied to baseline UBC, determined by linear regression analysis of binding data (peroxidase method) from 86 newborns, at total bilirubins of 12 mg/dL for sulfisoxazole and 10 mg/dL for benzoate. Sensitivity and specificity were determined with existing data. Results: Sulfisoxazole and benzoate increased UBC in artificial sera 2.1-fold and 4.1-fold, respectively, and in plasma (sulfisoxazole) 2.4-fold. Benzoate would increase baseline UBC from 0.29 to 1.19 μg/dL and sulfisoxazole from 0.36 to 0.86 μg/dL. The sensitivity and specificity of a UBC of 0.86 μg/dL for predicting kernicterus are 79% and 92% and for 1.19 μg/dL, 50% and 98%, respectively. Conclusion: Historic data predict that the unbound bilirubin above which kernicterus becomes likely lies between 0.86 and 1.19 μg/dL, in good agreement with existing information. (J Pediatr 2000;137:540-4)