Diabetes mellitus is nor a diagnostic criterion for Prader-Willi syndrome (PWS), but it is often found in PWS patients. Thr: etiology for diabetes mellitus in PWS may be related to the morbid obesity and consequent insulin resistance, because a decrease of oxytocin neurons and leptin resistance in PWS may cause hyperphagia, inducing obesity. However, treatment with growth hormone (GH) is beneficial for the majority of GH-deficient PWS children, because relative decreased fat mass and increased fat-free mass could prevent obesity and concomitant insulin resistance. Hypogonadism is thought to be due to hypogonadotrophic hypogonadism in a majority of PWS patients. Hypergonadotrophic hypogonadism secondary to cryptorchidism and its treatment is shown in other cases. Low luteinizing hormone and high follicle-stimulating hormone levels in PWS cases in young men with idiopathic oligospermia or in the early stages of puberty is less frequently reported. (C) 1999 Editions scientifiques et medicales Elsevier SAS.