A neurologically normal 3-year-old girl with Type I Crigler-Najjar syndrome was successfully treated with orthotopic liver transplantation. Preoperative serum bilirubin concentrations as high as 31 mg per dl were not diminished with phenobarbital or phototherapy. Bilirubin fractionation of duodenal bile prior to transplantation revealed 87.1% unconjugated bilirubin and 12.9% monoconjugates as determined by alkaline methanolysis-high-performance liquid chromatography. Postoperatively, the serum bilirubin concentration quickly fell to normal. Uridine diphosphate glucuronyl transferase activity in the recipient liver was not detectable. The gallbladder bile bilirubin concentration of 23.9 mg per dl was less than 15% of previously reported normal values. Since devastating kernicteric brain injury is the invariable outcome of Type I Crigler-Najjar syndrome, liver transplantation should be performed when phototherapy cannot maintain the serum bilirubin concentration at an unequivocally safe level.