The prevalence of ALS and its distribution in Finland was investigated on the basis of prevalence numbers by counties. On the prevalence day, 1 January 1973, 168 ALS patients were found alive. They were collected from hospital records and the registers of the National Pension Institute. The prevalence was 3.56 cases per 100,000 populations, 4.25/100,000 for men and 1.92/100,000 for women, which gave a male to female prevalence ration 1.5 to 1. The distribution of the disease was uneven in the country being more frequent in the southeastern counties. The disease was contracted earlier (at 51.9 years of age) and its course was slower (3.7 years up to the prevalence day) in the prevalence material than in cases derived from the mortality statistics (58.0 years at the onset and duration of 2.6 years, respectively). This is explained by the dropping out of some of the younger and/or more benign cases from the mortality statistics. This seems to be true also in other countries.