The information of 349 cases of multiple sclerosis, seen in a neurological department over a twenty-year period and followed up for a mean of nine years, was analysed by computerized data processing. The mean age at onset was 30.0 years for the remittent onset types (82 per cent cases) and 37.3 years for the progressive onset types (18 per cent cases). During the course of the disease the age of the 'pure relapse' stage was 29.2 years, of the relapse with sequelae stage 33.9 years and of the progressive phase 38.0 years. The interval between the first two relapses in the remittent-progressive type was important, the shorter the interval the sooner the progressive phase occurred. The relapses tended to increase in frequency before the progressive phase started. Using an actuarial graph, 50 per cent of cases could be expected to be moderately disabled (still ambulatory) in six years, and severely disabled (not ambulatory) in eighteen years and dead in thirty years. Combining a disability score and the duration of the disease prognostic factors could be studied. A late onset of the disease, a short interval between the first two relapses and the occurrence of the progressive phase were associated with a poor outcome. Sex of patient, the symptomatology of the initial relapses, and the constituents of the CSF had no prognostic value.