People with Prader-Willi syndrome exhibit infantile hypotonia and failure to thrive, genital hypoplasia, childhood-onset obesity, mental deficiency and behavioral abnormalities, hypogonadism, short stature, and characteristic dysmorphology. In over half the affected individuals, prometaphase chromosome analysis reveals a small interstitial deletion of chromosome 15q, del 15(q11-q12); with most of the remaining patients showing apparently normal chromosomes. Molecular genetic technology is currently being applied to the relevant region of chromosome 15 to determine if there is etiologic heterogeneity and to seek a consistent diagnostic marker. Diagnosis at this time is primarily based upon clinical criteria.