The results of segregation analysis applied to a family study of idiopathic torsion dystonia in Ashkenazi Jews are reported. The study is based on 43 probands (with age at onset prior to 27 years) from 42 nuclear families; pedigrees were extended systematically through all available first- and second-degree relatives, who were directly examined and videotaped. Final diagnoses were based on exam information and blinded videotape review. Segregation analysis demonstrated that the data are consistent with autosomal dominant inheritance with 30% penetrance. Recessive and polygenic inheritance were strongly rejected. There was no evidence for sporadic cases or new mutations. The high incidence and dominant inheritance of early-onset idiopathic torsion dystonia in Ashkenazi Jews suggests genetic homogeneity within this population, making it especially useful for linkage studies of this disorder.