Chronic fatigue syndrome is characterized by persistent, recurrent and unexplainable chronic fatigue not due to an ongoing period of effort, not relieved by rest, and is functionally disabling. Establishing this diagnosis requires the presence of at least 4 of the following associated symptoms: 1) patient report of a deterioration in short-term memory that is functionally limiting; 2) pharyngodynia; 3) cervical or axially lymphadenopathy; 4) muscle pain; 5) polyarticular pain without tumefaction or reddening; 6) headaches with new characteristics or of different severity; 7) unrefreshing sleep; 8) malaise following effort and lasting more than 24 hours (post-exertional malaise).