Age of onset and severity of idiopathic torsion dystonia (ITD) were studied in 100 British families containing 107 index cases and 79 secondary cases. Analysis of variance of these clinical features did not suggest that ITD is genetically heterogenous, and they were similar in Jewish and non-Jewish patients. Intrafamilial correlation for age of onset was low, particularly between parents and their offspring, suggesting that the ITD phenotype may be determined in part by nongenetic factors or an allelic modifying gene.