A group of 47 patients (multiple sclerosis (M.S.) diagnosed according to Schumacher's criteria) with age at onset over 40 years, was studied by statistical analysis and compared with a control group (100 M.S. patients with onset between ages 18 and 38). The following features appear peculiar in this group: --the most frequent initial symptoms are motor disturbances --the onset is generally monosymptomatic --the course is generally progressive and more severe than that of controls, as seen by Kurtzke's Disability Scale, relapse rate, interval between first and second attack --the course is more severe when the onset is progressive and characterized by motor disturbances.