A genetic study of idiopathic focal dystonias was undertaken by examining 153 first-degree relatives of 40 index patients with torticollis (14 patients), other focal cranial dystonias (16 patients), and writer's cramp (10 patients). Nine relatives with dystonia were identified in 6 families; 8 of these had symptoms such as clumsiness or tremor, but none were aware of any dystonia. A further 4 relatives, now decreased, were affected by history. Overall, 25% of index patients had relatives with dystonia. The results of segregation analysis suggested the presence of an autosomal dominant gene or genes with reduced penetrance as a common cause for focal dystonia. Segregation ratios were not significantly different from those ratios observed in generalized or segmental dystonia in the United Kingdom, and it is possible that a single autosomal dominant gene mutation is responsible for inherited dystonia in the majority of patients irrespective of distribution or severity.