This review summarizes the symptoms and signs seen in patients with chronic fatigue syndrome (CFS). It is based on the authors' experience with two cohorts of ∼510 patients with chronic debilitating fatigue and on the reported experience of other investigators with similar patients. The most characteristic symptoms of CFS are the sudden onset of an infectious-type illness, the subsequent chronic and debilitating fatigue, and postexertional malaise; many patients also have recurrent fevers, pharyngitis, adenopathy, myalgias, sleep disorders, and cognitive impairment.