A study has been undertaken to ascertain the changes in mortality from motor neurone disease (MND) and from multiple sclerosis (MS) in England and Wales and in the Republic of Ireland. During the 20 years 1968-1987, 16,077 deaths were reported as being primarily due to MND in England and Wales with a male/female ratio of 1.22. There has been an increase in MND deaths from 3,185 in 1968-1972 to 5,241 in 1983-1987. The increase occurred in the death rates in both sexes and in all age groups, but particularly over the age of 65. In contrast, there was no increase in MS deaths and the MS death rates fell below the age of 55 but increased over this age, evidence that MS patients are living longer. A similar but more marked increase in MND mortality, and a considerable fall in MS mortality, occurred in the Republic of Ireland. The increase in MND mortality is not due to an increase in the number of neurologists, as there has been little increase in their numbers. The highest MND mortality was in Social Class IIIN males - skilled non-manual workers.