All cases of ALS in Sweden during the period 1970-1983, i.e., 1961 cases, were compared with an age-stratified random sample of 2245 individuals from the Swedish population. On the basis of census information, the male cases were found to be heterogeneously distributed over occupational groups. Significantly more male cases than expected were found among office workers (OR = 1.8; 34 cases) as well as among farm workers (OR = 1.7; 56 cases). There was a cluster of male cases in agricultural work in one south-western county (OR = 3.4; 25 cases). Significantly more female cases than expected were medical service workers (OR = 1.7; 33 cases).