Two hundred and ninety cases of subacute sclerosing panencephalitis (SSPE) registered in England and Wales from 1970 to 1989 were followed at 6-monthly intervals. Male to female ratio was 2.8:1. Age at onset increased significantly over the period. Measles was recorded for 81% of cases; in nearly half this had occurred under 2 years. Measles vaccine was documented in 20 cases; in 10 measles was also documented and it could not be positively excluded in the remainder. The calculated risk of SSPE following measles was 4.0/100,000 cases compared with the risk after vaccine of 0.14/100,000 doses. Measles under 1 year carried a risk 16 times greater than measles over 5 years. There was an excess of cases in third and subsequent children. The incidence was higher in the northwest than in the southeast of the country. Survival time varied from 4 weeks to 16 years and was shorter when measles had occurred over the mean age of 2.5 years. Of the cases 9% had a history of mental retardation before the onset of SSPE. The incidence of SSPE has fallen following the reduction in measles resulting from vaccination. However, because of the median 8-year interval between measles and onset of SSPE, further cases arising from measles during the study period must still be expected, particularly in adolescents.