A country-wide study of the frequency of amyotrophic lateral sclerosis (ALS) was undertaken in Israel for the period 1960-1970. Israel was chosen for this study because of its excellent medical facilities and detailed demographic information. Moreover, the population includes representative groups from all parts of the world for comparison of frequency. A wide variety of motor system disease was screened in all hospitals, clinics, and chronic care facilities in the country, death certificates were reviewed and physicians with a neurological practice were contacted to derive a tentative list of cases. Only those who fit strict clinical diagnostic criteria or had autopsy confirmation were included in estimates of prevalence and incidence. On January 1, 1965, the mid-point of the study, 62 patients with ALS were living in Israel. The age-adjusted prevalence of ALS on that date was 3 per 100,000 population. The average annual age-adjusted incidence for the period 1960-1970 was 0.78 per 100000 population )0.86 in males, 0.46 in females; ratio 1.9:1). There was no appreciable change in trend of incidence over the study interval. Age-specific incidence rates were similar in native-born inhabitants of Israel, immigrants from Europe and immigrants from Afro-Asian countries. The range in age-adjusted incidence among subgroups of immigrants to Israel from various countries was 0.25 to 1.20 per 100000 population but small numbers precluded testing the statistical significance of these rather narrow differences. Mean age at onset was 55.4 years for males and 52.4 years for females. The mean age at death was 60.2 for males and 58.0 for females. The average annual mortality from ALS was 0.58 per 100000 population. There were no familial aggregates of ALS in Israel and autopsy data showed no neurofibrillary changes, granulovacuolar or inclusion bodies. There are only a few other population studies of ALS in different regions of the world. The average annual incidence in these other studies ranged from 0.4 to 1.4 per 100000 population. Thus, the incidence in Israel falls within this narrow range. The present study lends further support to the impression that ALS has a remarkably uniform geographic distribution with Guam and the Kii peninsula of Japan being the only known areas with significantly high rates. If an environmental factor contributes to the pathogenesis of ALS, the factor must also have a uniform geographic distribution.