We report the first family with adult onset cervical dystonia in which monozygotic twins and multiple family members are affected. In this family, the disease exhibits an autosomal dominant inheritance pattern, and all affected members have only cervical dystonia. Five members have definite cervical dystonia, and five others have possible cervical dystonia. Although identical genotypically, the twins demonstrate some phenotypic variation. Despite long follow-up, no family members have shown progression from focal to generalized dystonia. This family may prove valuable in identifying a gene locus for cervical dystonia and hence determine whether a single gene locus exists for hereditable focal dystonia and generalized dystonia.