• The age-standardized mortality from motor neuron disease in Sweden doubled from 1961 to 1985. The average annual rate during the period was 1.9 per 100 000 population. The male to female ratio was 1.2:1. The age-specific mortality rates had a peak at 70 to 79 years of age. When each birth cohort was followed up separately over time, the peak was less clear and in some cohorts the mortality rates increased continuously with advancing age. A significant increase of motor neuron disease among men was found in one Swedish county.