In order to further elucidate the genetics of Graves' disease, we studied two families with several affected members, as well as tested the degree to which HLA haplotypes were shared in affected sibpairs. Further, we sought to identify the disease related haplotypes by determining the haplotypes shared among affected parent-child combinations. In one family, two affected sibs differed at four possible parental HLA haplotypes; no evidence of recombination was observed which could account for the result. In the other family, five siblings were affected. Four out of the five affected sibs shared the maternal haplotype HLA-A11, Bw51, Cw5, Cw-, DRw5, Bfs, GLO1, whereas three shared the paternal haplotype HLA-A1, B8, Cw-, DRw3, BfS and GLO1. Looking at haplotype sharing, two pairs of sibs were found to be HLA identical, whereas the fifth sib shared one haplotype with one of these pairs but not with the other. Out of 14 (eight of our own and six from the literature) affected sibpairs examined, nine were found to be HLA identical and four shared one haplotype, suggesting that the contribution of both paternal haplotypes may be necessary for the susceptibility to the disease. Fourteen parent-child combinations were studied; in only three out of 13 in which the shared haplotype could be ascertained was the haplotype B8 positive; this distribution is similar to controls. However, of the remaining 10 combinations which did not share a B8 positive haplotype, five were B8 positive at one or the other of the non-shared haplotypes.