The total population under 25 years of age in one rural county in Sweden was screened in an attempt to find all cases of Prader-Willi syndrome and to establish prevalence rates for the disorder. 11 clear and five suspect cases were found. For the seven- to 25-year-old age-group the prevalence was 13 per 100,000 for clear cases only, but 20 per 100,000 if borderline cases were included. There was a slight over-representation of males, but it was possible that this was a result of under-representation of females in the very young age-groups, in whom hypogonadism may be difficult to diagnose.