The authors conducted a study of 518 amyotrophic lateral sclerosis patients identified between 1977 and 1979 and 518 controls to investigate putative risk factors for this disease. Occupations at risk of electrical exposure were reported more often by patients (odds ratio (OR) = 3.8, 95% confidence interval (CI) = 1.4-13.0) as were electrical shocks producing unconsciousness (OR = 2.8, 95% CI = 1.0-9.9). Although an overall excess of physical trauma associated with unconsciousness was observed in the amyotrophic lateral sclerosis patients (OR = 1.6, 95% CI = 1.0-2.4), the effect was inversely associated with duration of the unconscious episodes, suggesting an effect of recall bias. Only slight differences were found for surgical traumata to the nervous system. Parkinsonism was reported more often among first degree relatives of cases (OR = 2.7, 95% CI = 1.1-7.6). The frequencies of prior poliomyelitis or other central nervous system diseases were similar for patients and controls. Occupational exposure to selected toxic substances was similar for patients and controls except for the manufacture of plastics (OR = 3.7, 95% CI = 1.0-20.5), although few details of these exposures were provided. No differences in occupations with exposure to animal skins or hides were observed.