ARIAS , WOLFSON , LUCEY , AND McKAY TABLE II

Normal full-term infants may have transient unconjugated hyperbilirubinemia that rarely exceeds 5 mg per 100 ml during the first 3 to 5 days of life (1, 2). This so-called physiologic hyperbilirubinemia is believed to result from delayed development of the hepatic glucuronide conjugating system (3-5), particularly glucuronyl transferase (6). Numerous factors, such as hemolysis, infection, drugs, and prematurity influence the severity of unconjugated hyperbilirubinemia in newborn infants; however, many cases of severe neonatal unconjugated hyperbilirubinemia are observed for which no etiologic explanation is available (7, 8). The possibility that some of these cases may have a familial basis has not been previously suggested. Our attention was called to this possibility by observations made during the last 7 years on five mothers who had given birth to a total of 16 infants each of whom had severe transient unconjugated hyperbilirubinemia for which no adequate explanation was found. Three of these infants died of kernicterus, and one surviving child had quadriplegic cerebral palsy that probably resulted from kernicterus. Subsequent to preliminary reports of this syndrome (9, 10), three additional families became available for study. In 1957 Lathe and Walker demonstrated that serum from pregnant women and their newborn infants regularly inhibits the formation of conjugated bilirubin by rat liver slices in vitro (11). In 1958 we suggested that transient familial neonatal hyperbilirubinemia may result from increased amounts of a substance in the serum of certain