Characteristics are hypotonia, problems with feeding and thriving in the neonate and infant, later hyperphagia and severe obesity. Other findings are dysmorphic traits, hypogonadism, short stature, developmental delay, mental retardation and behavioural problems. Diabetes mellitus (NIDDM) is frequent in adults. Treatment is symptomatic. Prognosis is determined by obesity. PWS occurs almost always sporadically and is found in all ethnic groups and in both sexes. The epidemiology of PWS in Denmark is unknown. In 95% of cases with PWS cytogenetic and molecular genetic investigations show either deletion of the paternal chromosome 15q11q13 or uniparental maternal disomy of chromosome 15. Since 1992 150 bloodsamples of patients suspected for PWS have been investigated by cytogenetic and molecular genetic techniques at the John F. Kennedy Institute, DK-2600 Glostrup; deletion of the paternal chromosome 15 was found in 15 and uniparental maternal disomy of chromosome 15 in eight cases.