We wanted to establish the point prevalence of MS in Northern Ireland. Northern Ireland has been a known high-risk area for MS since the original work of Allison and Millar in 1951. Studies demonstrated a rising prevalence in 1951 of 41 (95% CI, 37 to 44), in 1961 of 57 (95% CI, 53 to 61), and in 1986 of 104 (95% CI, 85 to 128)/10(5) population for probable/early probable and latent disease. We surveyed four districts (area 2,030 km2, population 151,000). Case sources included Regional Neurology Service records, postal survey of general medical practitioners, hospital databases, MS charities, and MS respite facilities. We used the Poser and Allison and Millar criteria in diagnosis. From a provisional list of 402 patients, 254 (177 female, 77 male, ratio 2.3:1) were identified with definite or probable disease by the Poser criteria, prevalence 168.2/10(5) (95% CI, 147.5 to 188.9/10(5)) with 34 suspected patients and an overall prevalence 190.7/10(5) (95% CI, 168.7 to 212.7/10(5)). We interviewed and examined over 87% of these 288 patients. There were 246 patients classified as probable/early probable and latent by Allison and Millar, prevalence 162.9/10(5) (95% CI, 142.6 to 183.3/10(5)). Seventy-six probable or definite patients had onset between 1985 and 1992 (estimated incidence, 6.5/10(5)/yr). Age range of prevalent patients was 18 to 79 years (mean +/- SD, 49.2 +/- 13.3 years). Mean age at onset was 31.6 +/- 10.1 years. Overall, 12.5% had primary progressive disease and 20% had benign MS. Northern Ireland has an extremely high and rising MS prevalence. This may reflect a true increase in disease incidence or may be explained by increasing survival, improved case ascertainment, increased disease awareness, or improved laboratory and radiologic diagnosis.