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In order to establish the incidence and prevalence of type I spinal muscular atrophy (SMA Werdnig-Hoffmann disease) in North Dakota, we reviewed the death certificates for the past 8 years. Between 1980 and 1987 the prevalence of was 1.5 per 10,000. The incidence was 1 in 6,720. This suggests a carrier frequency of 1 in 41 in North Dakota with a gene frequency of 0.0122. In North Dakota, type I spinal muscular atrophy appears to be 3 to 10 times more common than in other locations.

Service providers for children in the state of North Dakota were surveyed to determine the prevalence rate of Rett syndrome (RS). Five patients with definite RS were identified in a population of 203,801 children (98,932 girls) 0-18 years of age yielding a prevalence rate of RS of 1 in 40,760 in North Dakota children. For girls the rate of RS is 1 in 19,786. Similar surveys of other states using comparable modes of ascertainment will be important as points of comparison for prevalence rates of RS.

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.

An institution for the mentally retarded was surveyed to determine the prevalence rate of Rett syndrome (RS). Four patients with definite RS and one with probable RS were identified in a population of 350, yielding a prevalence rate of 1 in 87, about 1% of institutionalized male and female patients with mental retardation. In this population of patients with severe and profound mental retardation (N = 297), 138 females were surveyed, suggesting a prevalence rate among females of 1 in 34 in an institutional population of persons with mental retardation. Surveys of institutions for persons with mental retardation may be an effective method to identify adults with RS.