Fatigue is a common symptom of multiple sclerosis (MS), but researchers now suggest that fatigue is a diagnostic sign of this disease in some individuals. In a study of 5,305 patients with newly diagnosed MS and with 3 years of prediagnosis health-care coverage, 1,534 (28.9%) had a fatigue-related disorder in the 2 years preceding diagnosis of MS. The researchers suggest careful history-taking and physical examination in patients who present with fatigue could identify transient neurological conditions that may herald MS.