Chronic Fatigue Syndrome (CFS) is a disorder characterized by unexplained, persistent, and sometimes debilitating fatigue. Diagnosis can be difficult because there are no objective clinical or laboratory findings associated with this disorder. The long term prognosis is favorable in many cases, although treatment options are limited.

The prevalence of CFS is unclear, partly because of difficulties in diagnosis. For example, in one study of 1000 consecutive patients in a primary care clinic, 8.5 percent had debilitating fatigue that had lasted at least six months without any apparent cause. Only 15 percent of these patients, however, satisfied the clinical definition for CFS described below.

Although many people think of CFS as a "disease of the 90s" this is not at all the case. Conditions whose symptoms match those of CFS have been described in the medical literature under a variety of names since the eighteenth century.

In the 1880s, the neurologist George Beard named and promoted the term neurasthenia. The famous nineteenth century physician Sir William Osler, writing in his Principles and Practice of Medicine in 1895, included this note in his description of neurasthenia: "in all forms there is a striking lack of accordance between the symptoms of which the patient complains and the objective changes discoverable by the physician."

In 1934, an outbreak of myalgic encephalitis at Los Angeles General Hospital formed an example of the "epidemic" form of this disease. Other such outbreaks have been called Iceland disease, Akureyri disease, and Royal Free disease. Regardless of name, however, all have resembled the currently described symptoms of CFS.