Recent surveys indicate a decreasing north-to-south gradient in the mortality and incidence rates of amyotrophic lateral sclerosis (ALS) in Italy, possibly indicating a different susceptibility to ALS in these populations. Piemonte, a region of northwestern Italy, experienced a considerable migration from other regions in Italy between 1940 and 1975; we therefore analyzed the effects of place of birth and migration upon the risk of developing ALS. Data on all ALS cases occurring in Piemonte during the period 1971-1990 were collected. Standardized incidence ratios (SIRs) for patients born outside Piemonte were calculated, using the Piemonte-born population as reference. A total of 962 ALS cases were identified during the study period, corresponding to a mean annual crude incidence rate of 1.37/100,000 population (95% confidence interval, 1.29-1.46). The SIRs of patients born in three southern Italian regions and of foreign-born persons were significantly higher than those of persons born in Piemonte and or other regions in Italy, and increased with age. This observation may be explained by an interaction between environmental and genetic factors or by selective migration.