To the Editor.—
In the article "Amyotrophic Lateral Sclerosis in a High Selenium Environment" by Kilness and Hochberg (237:2843,1977) and in a subsequent publicity release by the American Medical Association, it was implied that excessive toxic amounts of selenium were the cause for a cluster of four cases of amyotrophic lateral sclerosis (ALS), which occurred in a highly seleniferous, sparsely populated region of South Dakota within a ten-mile radius from each other. I would like to point out that clusters of ALS occur elsewhere in areas with normal or low selenium levels in the geochemical environment. It cannot be concluded from the available evidence that a high intake of selenium is a cause or the cause of ALS. The most frequently occurring error in scientific logic is the assumption that two phenomena that occur side by side may be, or must be, connected with each other as cause and effect.