The α synuclein gene (SNCA) has been implicated in autosomal dominant forms of Parkinsons disease. We screened 119 individuals from families with this rar e form of the disease for SNCA duplications by semiquantitative multiplex PCR. T wo patients had duplications, which were confirmed by analysis of intragenic and flanking microsatellite markers. The phenotype in both patients was indistingui shable from idiopathic Parkinsons disease and no atypical features were presen t, by contrast with reports of families with triplication of the same gene. Thes e results indicate that SNCA is more frequently associated with familial Parkins ons disease than previously thought, and that there is a clear dosage effect a ccording to the number of supernumerary copies of this gene. The α synuclein gene (SNCA) has been implicated in autosomal dominant forms of Parkinson’s disease. We screened 119 individuals from families with this rar e form of the disease for SNCA duplications by semiquantitative multiplex PCR. T wo patients had duplications, which were confirmed by analysis of intragenic and flanking microsatellite markers. The phenotype in both patients was indistingui shable from idiopathic Parkinson’s disease and no atypical features were presen t, by contrast with reports of families with triplication of the same gene. SNCA is more frequently associated with familial Parkins on  s disease than previously thought, and that there is a clear dosage effect a ccording to the number of supernumerary copies of this gene.