Three adult sisters with previously unrecognized Prader-Willi syndrome (PWS) demonstrated the six diagnostic features of this congenital condition: neonatal hypotonia, hypomentia, hypogonadism, obesity, short stature, and dysmorphism. Detailed endocrine investigations were performed, including ovarian biopsy in the propositus. HLA genotype A2 was present in each patient. The normal high-resolution prometaphase karyotypes indicated heterogeneity; the absence of the deletion 15q12 frequently found in patients with sporadic PWS distinguished this sibship as representing a possible autosomal recessive type of PWS. Current evidence suggests that the diagnosis of PWS may be often overlooked. Increased clinical awareness of the features of PWS should result in prompt diagnosis and optimal management of affected patients, together with increased understanding of this enigmatic condition.