BACKGROUND Multiple sclerosis (MS) is the most frequent demyelinating disorder of the central nervous system. It mainly affects young adults and it has been calculated that between 20% and 30% are of late onset (after the age of 40), presenting clinical features, a clinical course and prognosis which are specific to this disorder. METHODS Between 1985 and 1994 we studied 297 cases of MS (diagnosed according to the criteria of Poser) and found that in 20.5% the illness had started when the patients were over the age the 40. CONCLUSIONS Certain aspects of the clinical features and course of the disorder were compared, taking the group of late onset cases and a randomized sample of 100 cases of early onset, showing that the symptoms and pyramidal signs were more frequent in the late onset group, whilst sensory and visual signs were commoner in the early onset group. In both groups there were more females. MS was defined according to Poser's criteria in 78.7% and 77% respectively. Chronic, primary and secondary progressive forms predominated in tire late onset group, whilst in the group with onset before the age of 40 there was a predominance of the exacerbating-remitting form.