The clinical syndrome of kernicterus in erythroblastosis fetalis is reviewed. Data are presented which indicate that there is a tendency for the familial occurrence of kernicterus in siblings. Kernicterus is shown to be directly related to the intensity of maternal sensitization, and to the degree of immaturity of the infant. It is more common in male infants than in female. Evidence is presented that the occurrence of kernicterus in a baby with erythroblastosis fetalis cannot be accurately predicted on the basis of degree of clinical illness at birth, nor on the basis of laboratory examination of the baby's blood at birth. The possible relationship of kernicterus to hyperbilirubinemia and to other predisposing factors in erythroblastosis fetalis is discussed.