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 approximately 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.