Amyotrophic lateral sclerosis in a high selenium environment.

During a ten-year period, four cases of amyotrophic lateral sclerosis have been found in a sparsely populated county (population 4,000) in west-central South Dakota. The patients were unrelated male farmer-ranchers between 57 and 66 years of age, living within a 15-km radius of each other. The cases occurred in a region where naturally occurring selenium toxication is endemic in farm animals.