Fatigue is a frequent symptom in multiple sclerosis (MS) that can interfere with a patient's daily functioning. The cause of MS fatigue, its clinical characteristics, and its relationship to other symptoms remain poorly understood. Structured interviews were conducted with 32 patients with MS and 33 normal healthy adults. Fatigue proved to be both more frequent and more severe among the patients with MS. Multiple sclerosis fatigue was unrelated to either depression or global impairment. Multiple sclerosis fatigue appears to be a distinct clinical entity, often disabling, that can be distinguished from normal fatigue, affective disturbance, and neurologic impairment.