Chronic fatigue syndrome (CFS) gained legitimacy in 1988 when a case-definition was proposed by a multidisciplinary group ( l) assembled by the Centers for Disease Control and Prevention (CDC) and has been the focus of intense public attention and scientific research ever since. According to the latest revision of the case-definition (2), CFS is said to be present when an individual has persistent or relapsing fatigue that is of new or definite onset, has remained unexplained after a thorough clinical evaluation, is not the result of ongoing exertion, is not substantially alleviated by rest, and produces significant reduction in previous levels of occupational and social activities. In addition, at least four of the following eight symptoms must have persisted or recurred for at least six months: substantial impairment in short-memory and concentration, sore throat, tender cervical or axillary lymph nodes, muscle pain, multijoint pain, headaches of a new type, unrefreshing sleep, and postexertional malaise lasting more than 24 hours. The definition makes clear that the CFS diagnosis cannot be substantiated by abnormalities found on physical examination and that no laboratory tests or imaging procedures are required or useful for the positive identification of CFS. However, judicious and reasonably complete testing should be used to exclude other physical diagnoses and a psychiatric evaluation or structured psychiatric interview must be performed whenever the patient's history and present symptoms indicate the possibility of a psychiatric disorder.