A prevalence study of Prader-Willi syndrome (PWS) has been conducted in North Dakota. All pediatricians, neurologists, child psychiatrists, psychologists, and clinical geneticists were surveyed. The state's comprehensive evaluation center, the state hospital, the state institution for the mentally retarded, and group homes for the developmentally disabled, including one for persons with PWS, were also contacted. Seventeen patients were identified, 8 males, 8 females, and one patient whose sex was not specified. This suggests a prevalence rate of 1 per 16,062 in North Dakota.