The clinical expression of dopa-responsive dystonia (DRD) was found to be different in a pair of affected monozygotic twins. An earlier onset was associated with a more disabling course of disease. Whereas monozygosity was genetically proven, the search for pathogenic mutations in the GTP-cyclohydrolase-1 gene was negative. The contribution of environmental factors appeared minimal. Intrafamilial variability of DRD phenotype may be related to yet unknown non-Mendelian epigenetic or proteomic factors.