Fibromuscular dysplasia (FMD) is an arterial occlusive disorder of young people that reportedly has affected more than one sibling in several families. A formal pedigree analysis was conducted in 20 families in which at least one member had documented FMD. Clinical symptoms compatible with the disorder were sought in all available family members. In eight families (40%), only the index patient seemed to be affected. The other 12 families contained between one and 11 other relatives who appeared to have FMD. Vertical transmission of the disease was demonstrated repeatedly. There was no consanguinity, and both sexes were equally afflicted. The inheritance pattern for FMD in this investigation was most consistent with an autosomal dominant trait with variable penetrance.