A set of male monozygotic twins concordant for Huntington's disease is described. The monozygosity of the twins and the diagnosis of Huntington's disease are well established. The twins are now age 30, and although the severity of their chorea differs, they have a similar degree of mental deficit. This family is of additional interest because the daughter of one of the twins has childhood Huntington's disease, and the mother of the twins had the adult-onset rigid variant of the disease. Such unusual families afford some insight into the variability of the clinical manifestations of this hereditary disease.