At least three types of congenital nonhemolytic unconjugated hyperbilirubinemias, including the rare Crigler-Najjar (CN) diseases (Types I or II) and Gilbert's syndrome (affecting 6% of the population) are associated with either absent or reduced hepatic UDP-glucuronosyltransferase (transferase) activity towards the potentially toxic endogenous acceptor, bilirubin. Here, we review the biochemical studies associated with these deficiencies. Accumulated evidence from studies with an animal model of CN Type I syndrome, the Gunn strain of hyperbilirubinemic rats, suggested that multiple isozymes are absent. These confounding observations have been clarified by a flurry of reports which have revealed the molecular basis for the complex disease phenotype in the Gunn rat and by the isolation and description of a novel human gene complex, UGT1, which encodes multiple and independently-regulated transferase isozymes that contain identical carboxyl terminal regions (246 amino acids). Finally, we discuss the implications of the gene organization and genetic defects determined for four different CN Type I individuals as a basis for a model which explains the inheritance pattern and genotypes of other familial unconjugated hyperbilirubinemias.