Patients with motor neurone disease resident in the three counties of South Glamorgan, Mid Glamorgan and Gwent (total population 1,394,400) from 1/11/91 to 31/12/92, were ascertained through general practitioners and neurology department records: 62 out of 71 (51 male, 20 female) index cases ascertained agreed to be visited. These individuals were clinically reviewed, had a detailed family history taken and, after discussion, had a blood sample taken for testing for the X-linked bulbospinal muscular atrophy mutation. The period prevalence of motor neurone disease was 5.02/100,000 with a mean age of onset of 59.4 (S.D. 12.9) years. One 66-year-old male was found to have the X-linked bulbospinal muscular atrophy mutation. A single, definite first-degree relative pair was identified, of whom only one was included in the data. The index cases had a total of 131 offspring and 173 siblings. The risk estimates, by current age, for these first-degree relatives of patients with apparently sporadic motor neurone disease decreased from 1 in 85 at age 20 years to 1 in 1683 at age 80 years. These estimates require confirmation in an independent sample, as they are based on a single "event".