Multiple sclerosis (MS) is the most common idiopathic inflammatory disease of the central nervous system. The distinction between MS and other benign or fulminant inflammatory demyelinating disorders is based on quantitative, rather than qualitative, differences in chronicity and severity. Primary progressive MS may differ from relapsing-remitting MS in MRI lesion frequency, immunogenetic profile, responsiveness to immunosuppressive treatment, and histology. In 60% of patients, MS begins as a relapsing-remitting disease and evolves secondarily into a progressive neurological illness. Life expectancy is not substantially altered in patients with MS, particularly in the early years of the illness. The rate of suicide has been reported to be increased sevenfold in MS patients. Up to 40% of patients with attacks severe enough to render them nonambulatory may not recover. At 15 years from MS onset, 50% of patients are disabled to the point at which they at least require a cane to walk a half block. Early age at onset, female sex, relapsing-remitting course at onset, and perhaps optic neuritis or sensory symptoms at onset and relatively few attacks in the first two years are associated with a favorable course.