Investigation of 19 unrelated patients with Gilbert's syndrome and family studies in 21 first degree relatives of 7 of these patients are in agreement with an autosomal-dominant mode of inheritance with incomplete penetrance. The calculated penetrance in our study is 57%. HLA typing (locus A, B, C) showed a slight, insignificant increase in antigens A11 and BW35. Furthermore, this syndrome did not segregate in parallel with certain haplotypes within families.