The uneven worldwide distribution of multiple sclerosis has been of interest to epidemiologists, neurologists and statisticians for over a century, prevalence rates for the disease apparently being determined by variations in age, gender, geography, race and ethnic group. Northern Ireland has been recognised as an area of high MS prevalence since the truly seminal work of Allison and Millar almost 50 years ago. The most recent study in Northern Ireland was undertaken in 1996 and involved the neighbouring districts of Coleraine, Ballymena, Ballymoney and Moyle (population, 151,000). Overall, 254 definite and probable cases were identified (prevalence: 168.2/100,000) with a further 34 suspected cases (overall prevalence: 190.7/100,000). Females predominated (ratio, 2.1:1) and the average age at onset was 31.6 years. The highest age specific prevalence rate for females was in the 35-44 years old age group (519.6/100,000) and for males was in those aged 55-64 (292.3/100,000). The spectrum of disability was broad and 20% could be considered to have relatively "benign" disease. These figures sadly confirm that Northern Ireland has one of the highest and rising MS prevalence rates in the world and implies an enormous potential for societary costs.