Fibromyalgia (FM) is a syndrome characterized by widespread musculoskeletal pain and tenderness at specified sites, fatigue, and unrefreshing sleep. The American College of Rheumatology (ACR) has recently accepted diagnostic criteria for FM. The ACR criteria have high diagnostic specificity, sensitivity, and accuracy. Laboratory investigations show a disturbed microcirculation in painful muscles, a decrease in adenosine triphosphate and phosphocreatine, and a reduced relaxation rate. Pain analyses indicate that the pain is nociceptive. A characteristic physiological sleep disturbance has been described that is correlated to the symptoms. The etiology of FM is not known. The etiology may be different in different patients. FM is a clinical entity, but should be regarded as a syndrome rather than a disease.Key words: pain, muscular pain, fibromyalgia, fibrositis.