Motor neuron disease (MND) is used in this paper as the generic label, encompassing the clinical variants of amyotrophic lateral sclerosis (ALS), progressive myelopathic muscular atrophy (PMMA), and progressive bulbar palsy (PBP). ALS is limited to instances of anterior horn cell plus pyramidal tract involvement. When only anterior horn cell lesions are inferred, either PMMA or PBP is used, depending on the levels of involvement; when both cord and brain stem are affected. PBP is the designation. Mortality data on MND have been available for a number of countries since 1949. The coding used under international rules has varied considerably over this interval. Before 1969, hereditary muscular atrophies were included. Since 1979, no subdivision by type of MND is possible. International death rates for MND have all been rather close to 1 per 100,000 population per year, though perhaps nearer to 1.4 on the average in recent years. There has been an increasing proportion of MND deaths coded to ALS between 1949 and 1977. There is no notable geographic variation among countries, nor within countries such as the U.S. and Denmark. A slight upward trend in death rates over time in the U.S. is matched by a slight decrease in Denmark. Death rates from all sources indicate a male preponderance for ALS or MND as a whole, at about 1.5 to 1, male to female. There is also a consistent predilection by age, with few deaths under age 50 or so and a clear maximum in age-specific death rates at about age 70. This holds for both sexes. In the U.S., there is also a white-nonwhite difference, with a ratio of about 1.6:1 but with age and sex differences similar to whites. Average annual incidence rates from among white occidental populations range mostly between 0.6 and 1.8 per 100,000 population for MND and about 0.8 and 1.5 per 100,000 for ALS. Again a male predilection is seen. There is a clear maximum in age-specific incidence rates at about age 65 in all surveys except that of Rochester, Minnesota, where the age-specific rate for those 75+ years of age is apparently higher than that for those age 65 to 74. Incidence rates, then, are quite similar one land to another. A reported deficit in Mexico may reflect case-selection bias. An excess among Filipinos on Hawaii seems more a function of population age-distributions than a true racial or ethnic difference. Prevalence rates from outside the Orient range from about 1 to 7 per 100,000 population for MND and about 2 to 7 for ALS. Those surveys more likely to be reasonably complete provide ALS prevalence rates of about 4 to 6, and an overall estimate of ALS prevalence of some 5 per 100,000 population is a reasonable figure. In the Orient, most of the MND prevalence rates fall within the same range as in the occident, except for two areas of the Kii peninsula of southern Honshu, Japan, where the reported prevalence rates are some 100 to 200 per 100,000 population. These cases are similar to the Guamanian ALS, both clinically and pathologically...