Fibromylagia is a painful musculoskeletal disorder composed of core features that are always present (wide-spread pain and tenderness), characteristic features that are present more than 75 per cent of the time (fatigue, nonrefreshed sleep, and morning stiffness), and common features that are present more than 25 per cent of the time (for example, paresthesia, irritable bowel syndrome, functional disability). The syndrome is common, occurring in 2.1 per cent of family practice clinic patients, 5 per cent of general medical patients, and 10 to 20 per cent of rheumatic disease clinic patients. Evolving diagnostic criteria permit identification of patients for clinical and research purposes.