Fatigue is one of the most common complaints among patients seen in the primary care setting. Chronic fatigue syndrome, which has recently been called chronic fatigue immune dysfunction syndrome, is distinctive, with an abrupt onset of symptoms that wax and wane for at least six months. Usually there is low-grade fever, pharyngitis and tender, but not enlarged, lymph nodes. The fatigue can be disabling and is often made worse by physical activity. Some patients with this disorder have also been found to have highly characteristic immunologic abnormalities. Treatment can be rewarding and is based on patient education and support, exercise and symptomatic therapies for abnormal sleep patterns, musculoskeletal pain and other symptoms.