Background 30 years ago very high multiple sclerosis (MS) prevalence rates were recorded in northern Scotland. A prevalence study was repeated in Aberdeen, Orkney and Shetland to see if prevalence rates had changed, assess which factors affect prevalence and record disability status.

Methods Hospital, general practice and laboratory records were searched to identify prevalent MS patients (alive and registered with a participating general practice on 24 September 2009). Records were reviewed to confirm diagnoses applying Poser definite and probable and McDonald diagnostic criteria. Disability status (Expanded Disability Status Scale) was recorded from records and questionnaires. Rates were standardised to the Scottish population.

Results 590 patients were found (Aberdeen 442, Orkney 82, Shetland 66). Mean age and disease duration were 53 and 19.4 years, respectively. The standardised prevalence rates for Poser probable/definite MS per 100 000 were: combined area 248 (95% CI 229 to 269), Orkney 402 (95% CI 319 to 500), Shetland 295 (95% CI 229 to 375) and Aberdeen 229 (95% CI 208 to 250). McDonald diagnostic criteria gave a lower prevalence (202, 95% CI 198 to 206). Prevalence was highest in women (2.55:1, 95% CI 2.26 to 2.89) with about 1 in 170 women in Orkney affected. Prevalence was lowest in the most deprived socioeconomic group. 45% had significant disability (Expanded Disability Status Scale ≥6).

Conclusion The prevalence of MS has increased in the overall area, most markedly in Orkney, then Shetland, over the past 30 years. This increase could be due to a number of factors, but rising incidence as reflected by a rising sex ratio, influenced by gene–environment interaction, is the most likely. Orkney has the highest prevalence rate recorded worldwide.