Fibromyalgia [FMS] is a multisystem illness with defining features located in the musculoskeletal system. Accumulating evidence derived from heart rate variability analysis and coming from different groups of investigators demonstrates that autonomic [sympathetic] nervous system dysfunction is frequent in patients with FMS. Such dysautonomia can be characterized as relentless sympathetic hyperactivity with hyporeactivity to stress. It is proposed that this type of dysautonomia explains the multisystem manifestations of FMS. Its defining features: generalized pain and tenderness at palpation in specific anatomical points are explained through the mechanism known as "sympathetically maintained pain." (C) 2002 by The Haworth Press, Inc. All rights reserved.