The association between chromosome abnormalities and the Prader-Willi Syndrome (PWS) was first tabulated by Hawkey and Smithies. 1976. These authors listed 8 reports which suggested to them that a D Group chranaome was commonly involved and they urged that banding should be done in these cases to elicit the nature ofthe D group chromosome. Since that time agreat deal ofwork hasbemdareonthe cytosenetia ofthe PWS and chromosome 15 has clearly been shown to be the D group chromosome involved This report brings up to date the cytogenetic data on the PWS.