The outcome of multiple sclerosis (MS), assessed according to the Kurtzke Disability Status Scale (DSS), was reviewed in 1,099 consecutive patients followed in London, Canada, between 1972 and 1984. A geographically based subgroup of 196 patients representing 90% of Middlesex County MS patients as well as a group of 197 patients seen from onset of disease were separately analysed. The clinical course was progressive from onset in 33% of the total population and in 28% of the Middlesex County subgroup. Of those with duration of 6-10 yrs, 30-40% with initially remitting disease developed progressive MS. The cross-sectional distribution of disability was bimodal with peaks at DSS 1 (no disability) and DSS 6 (assistance required for walking). Actuarial analysis showed that the median time to reach DSS 6 from onset of MS was 14.97 +/- 0.31 yrs in the total population and 9.42 +/- 0.44 yrs in the "seen from onset' subgroup. Survival was minimally altered; 87% of patients followed up to 40 yrs were still alive, although ascertainment of cases with this duration of MS was incomplete. Data describing the rate at which disability develops after the onset of a progressive phase of MS are also presented. The implications of these data in planning and interpretation of clinical therapeutic trials are discussed.