Mutations in the DYT1 gene cause idiopathic torsion dystonia (ITD) transmitted in families as an autosomal dominant trait with incomplete penetrance. The most common mutation, 946delGAG, has been observed in populations with different ethnic and geographic origins. We have investigated 40 individuals from 22 unrelated families with ITD originating from the Land of Valencia, Spain, for the presence of this mutation and we found 5 patients and 6 unaffected subjects from 4 families who were carriers of the mutation. This finding indicates that 18% of families may be diagnosed as DYT1 and that penetrance is reduced. We detected two different geographic and linguistic origins of the Valencian families. However, by haplotype analysis using D9S1260, D9S1261, D9S63 and D9S1262 as flanking markers, we demonstrated that all affected and unaffected carriers shared a common chromosome confirming identical origin of the mutation in the four families. We postulate a unique origin for the 946delGAG mutation in the Land of Valencia and, based on linguistic criterion, we propose that the mutation might have occurred at the beginning of the second millennium. Genetic analysis of another family from Castilla-La Mancha showed a different haplotype segregating with the disease, suggesting that at least two distinct mutational events for the 946delGAG mutation have occurred in Spain.