The families of three patients with Crigler-Najjar hyperbilirubinaemia, type 2, whose plasma bilirubin levels had responded to phenobarbitone treatment, were investigated. All the parents and several relatives had mildly raised bilirubin levels. It is suggested that this condition may be an example of genetic heterogeneity and that the propositi had inherited two different abnormal genes. The separation of the Gilbert and type 2 Crigler-Najjar syndromes is at present arbitrary.