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.