A patient with Prader-Labhart-Willi syndrome (PLWS) was found to have mosaic partial trisomy 15: 46,XY/47,XY, + del(15) (pter leads to q1.3:) in both lymphocytes and fibroblasts. Thus, another novel aberration is added to the spectrum of chromosome abnormalities seen in this syndrome. The spectrum includes deletion of the short arm of chromosome 15, interstitial deletion of 15q1.2, inverted duplication of 15p (tetrasomy 15p), partial trisomy 15 different from that encountered in this patient, and a variety of aberrations involving other chromosomes. A hypothesis that the chromosome aberrations are due to a presumed gene for the PLWS may have merit and could be tested in the laboratory by exposing chromosomes of patients with PLWS to mutagens to search for secondary chromosome derangements.