A 16-year-old boy with Prader-Labhart-Willi syndrome (PLWS) had hypotonia, feeding difficulties, failure to thrive, strabismus and bilateral inguinal hernias with cryptorchidism during infancy followed by hyperphagia, marked early-onset obesity with insulin-dependent diabetes mellitus and necrobiosis lipoidica diabeticorum, short stature, hypogonadotropic hypogonadism and some of the facial characteristics of the individuals with the PLWS. IQ is estimated around 90. Cytogenetic studies showed mosaicism: 45,X, t(Y;15) with partial deletion 15 (15pter----15q12); 46,X, t(Y;15), dic (15)(15pter----15q12::15q12----15pter) and 47, X, t(Y;15), dic(15), dic(15). The dic(15) was bisatellited, NOR-positive on both arms and represented inv dup(15). Thus, the 2 lines with the dic(15) showed partial trisomy 15 (15pter----15q12) and partial pentasomy 15 (15pter----15q12), respectively. The cell line ratios were different in lymphocyte and fibroblast cultures. The unique cytogenetic findings in this patient, the reports of a variety of chromosome 15 aberrations in PLWS, as well as aberrations of other chromosomes, suggest that the condition is a contiguous gene syndrome rather than an aneuploidy syndrome.