We report a prevalence study of multiple sclerosis (MS) in the town of Westlock and surrounding county of Westlock, in Alberta, Canada. The prevalence rate for clinically definite MS on January 1, 1991, was 200/100,000. The average annual incidence rates for patients living in the area at onset were 1.91/100,000 for 1950-1959, 2.85/100,000 for 1960-1969, 3.82/100,000 for 1970-1979, and 7.26/100,000 for 1980-1989. Forty-eight percent of the patients were relapsing-remitting. Sixty percent were still walking without assistance. The female-to-male ratio was 1.4:1. Mean current age was 47, age at onset 30, and duration of illness 18 years. The majority of patients (40%) experienced multiple symptom onset. Forty percent were of single ethnic origin (primarily British); the remainder were predominantly north European combinations. Twenty-four percent of patients reported another MS relative, six first-degree and one second-degree relative.