We describe a young woman with Prader-Willi syndrome (PWS) due to a mosaic imprinting defect. Three independent assays revealed a reduced proportion of nonmethylated SNURF-SNRPN alleles in peripheral blood DNA: methylation-specific PCR followed by denaturing high-performance liquid chromatography (MSP/DHPLC), methylation-sensitive restriction enzyme analysis and methylation-specific real-time PCR analysis. Microsatellite analysis and fluorescence in situ hybridisation revealed apparently normal chromosomes 15 of biparental origin. Based on the MSP/DHPLC and real-time PCR results, we estimate that approximately 50% of the patient's blood cells have an imprinting defect and 50% of the cells are normal. Apart from a rather normal facial appearance, the proband has typical features of PWS.