A survey of multiple sclerosis (MS) in the Cambridge Health District has identified 374 cases in a population of 288,410, giving a prevalence of 130 per 100,000. A total of 322 cases (86%) had either clinically definite or probable multiple sclerosis on 1 July 1990 (112 per 100,000) and 52 cases (14%) had suspected multiple sclerosis (18 per 100,000.) The incidence during 1989-91 was 5.94 per 100,000 per year. The prevalence figure is higher than in recent surveys from other southern parts of the United Kingdom, but correction for the age and sex characteristics of the at risk population eliminates these differences. The overall prevalence of multiple sclerosis is probably between 108 and 120 per 100,000 in the southern United Kingdom.