Chronic fatigue syndrome: A debilitating and complex disorder characterized by profound fatigue of six months or longer duration that is not improved by bed rest and that may be worsened by physical or mental activity. Persons with chronic fatigue syndrome (CFS) most often function at a substantially lower level of activity than they were capable of before the onset of illness. In addition to these key defining characteristics, patients report various nonspecific symptoms, including weakness, muscle pain, impaired memory and/or mental concentration, insomnia and post-exertional fatigue lasting more than 24 hours. In some cases, CFS can persist for years.

The cause or causes of CFS have not been identified and no specific diagnostic tests are available. Moreover, since many illnesses have incapacitating fatigue as a symptom, care must be taken to exclude other known and often treatable conditions before a diagnosis of CFS is made. In general, in order to receive a diagnosis of CFS, a patient must:

1. Have severe chronic fatigue of six months or longer duration with other known medical conditions excluded by clinical diagnosis, and
2. Concurrently have four or more of the following symptoms:
   • substantial impairment in short-term memory or concentration,
   • sore throat,
   • tender lymph nodes,
   • muscle pain,
   • multi-joint pain without swelling or redness,
   • headaches of a new type, pattern or severity,
   • unrefreshing sleep, and
   • post-exertional malaise lasting more than 24 hours.

Increasing activity and physical exercise have been reported to help ease the symptoms of fatigue in some patients with CFS, improving their quality of life and ability to function. Patients with CFS are sometimes treated with immune therapy, corticosteroids, antidepressants and other pharmacological agents or supplements, and complementary therapies. The value of these treatments is currently unproven. Some treatments have been shown to be of no value for CFS, among them dehydroepiandrosterone (DHEA), high colonic enemas, and kutapressin, a substance made from pig's liver.

Also known as Chronic Fatigue and Immune Dysfunction Syndrome (CFIDS) and as Myalgic Encephalomyelitis (ME).