A sample of hospitalized MS patients was selected according to clinical and demographic criteria with the aim of establishing prognostic factors. The sample included 52 patients with first hospitalization from 1 January, 1975, to 31 December, 1976. At follow-up after 12 years a malignant course was observed in 33 patients (death in 13, severe disability in 20 patients). The malignant course was related to age at onset (greater than or equal to 35 years) and higher disability, progressive course and cerebellar symptoms at onset. One half of patients with a relapsing-remitting course entered into a progressive phase of the disease after a mean duration of 7.3 years.