From 1962 to 1984, age-specific mortality for motoneuron disease (MND) in the United States rose in all demographic groups over the age of 40. The increase was seen in both men and women, and both whites and non-whites, and was most pronounced in the elderly (eg, 378% in white women aged 80-84 years). Men were at 50% higher risk than women, and whites had twice the risk of non-whites. These increases may reflect an improvement in case ascertainment, but they also seem to show a true rise in the incidence of MND, particularly among the elderly. Such an increase suggests an environmental aetiology.