Two case-control studies of motor neuron disease that involved 712 cases and 158 cases, respectively, showed that (1) mechanical injuries were two to three times more frequent in both sexes, heralding amyotrophic lateral sclerosis, progressive bulbar paralysis, and progressive muscular atrophy; (2) the head, neck, spine, and the extremities were more often traumatized; (3) traumatized parts were not correlated with the initial manifestation of the disease; and (4) more males were traumatized, but males still predominated among uninjured cases. These results suggested that mechanical injuries were not the cause, but probably one of the risk factors of the disease. No association was observed with smoking, drinking, residence, home space, drinking water, animals, experience as a war prisoner, stay on Guam, parental consanguinity, measles, polio, mumps, tuberculosis, rheumatism, prothesis of the total teeth, shell splinters retained in the body, occupational exposures to radiations, chemicals, or gases, atomic bombings, electric injuries, surgical operations, and occupations.