An epidemiological survey of amyotrophic lateral sclerosis (ALS) based on 83 patients living in the province of Florence (central Italy) showed an incidence of 0.714 X 100,000 inhabitants and a prevalence of 2.142 X 100,000 inhabitants. The disease was found to be more prevalent in males (sex ratio 1.3--1). Average age of onset was 59 years (57 +/- 4 for males and 61.6 +/- 3 for females). No particular geographical distribution was noted. 59 % of the patients presented the conventional amyotrophic form, while 10 % and 30 % were those with the bulbar and polyneuritic type, respectively. A survey of the social and economic status showed ALS to be more frequent in manual workers (P less than 0.001). Among all patients 31 % presented evidence of trauma; however only in 15 % of them could the trauma be chronically and topographically related to the onset of ALS. The presence of other diseases associated with ALS was examined but the combination found may be only casual.