We report on a white man with Prader-Willi syndrome (PWS) and del(15)(q11.2q13), confirmed by fluorescence in situ hybridization (FISH), who had hyperlipidemia, insulin-dependent diabetes, and the early onset and rapid progression of diabetic retinopathy and nephropathy within 4 years after diagnosis of diabetes. The spectrum of glucose intolerance in patients with PWS is discussed, as well as those references which suggest that the prevalence of hyperlipoproteinemia in this condition may be greater than previously recognized. We suggest the need for clarification of both the prevalence and types of hyperlipoproteinemia, as well as the pathophysiology of glucose intolerance and correlation with molecular cytogenetic findings. We also encourage careful monitoring for diabetic complications to further clarify the prevalence and possible accelerated course of microvascular lesions.