The mortality and prevalence of ALS in the various countries of Finland was studied. The work was based on death certificates derived from a 10-year period from 1963 to 1972, and altogether 421 cases were found. The mean duration of the disease was 2.7 years, and the mean age at death was 61.2 years. The average annual mortality rate was 0.91 per 100,000. The male to female ratio was 0.87 to 1, males outnumbered females only in age groups under 65. Some clustering of the cases seems to be taking place in the south-eastern part of the country. The rural to urban distribution of the patients' places of birth and domiciles did not differ markedly from that of thw whole population. After ALS itself pneumonia was the most common direct cause of death. Of other significant conditions coded in the death certificates schizophrenia and cancer did not occur more often than could be expected by chance. No evidence of inheritance of the disease was found.