The Sonya Slifka Longitudinal Multiple Sclerosis Study follows a population-based cohort of approximately 2000 people with multiple sclerosis (MS) to study demographic and clinical characteristics, course of illness, utilization and cost of health services, provider characteristics, use of MS specialists and disease modifying agents, and neurologic, economic and psychosocial outcomes. This report describes the study methodology, presents baseline demographic and clinical data, and evaluates the representativeness of the sample. A stratified random sample of persons with established and recently-diagnosed MS selected from the National Multiple Sclerosis Society (NMSS) mailing lists was supplemented with recently-diagnosed patients recruited through systematic nationwide outreach. Baseline data were collected by computer-assisted telephone interviews derived from standardized instruments; data collection continues at six-month intervals. The cohort was comparable to population-based and clinical samples with respect to demographics, course, relapse rate, symptoms, and severity of disability. Almost two-thirds of the cohort needed help with activities of daily living, three-quarters were limited in work or other activities, and half had emotional problems that compromised quality of life. The Slifka Study cohort is broadly representative of the MS population and the database can be used to address questions not answered by natural history studies, clinical databases, or population-based surveys.