The following is excerpted from THE FIBROMYALGIA NETWORK:

"The cause of fibromyalgia and chronic fatigue syndrome remains elusive, but there are many triggering events thought to precipitate its onset. A few examples would be an infection (viral or bacterial), an automobile accident or the development of another disorder, such as rheumatoid arthritis, lupus, or hypothyroidism. These triggering events probably don't cause FMS, but rather, they may awaken an underlying physiological abnormality that's already present in the form of genetic predisposition.

What could this abnormality be? Theories pertaining to alterations in neurotransmitter regulation (particularly serotonin and norepinephrine, and substance P), immune system function, sleep physiology, and hormonal control are under investigation. Substance P is a pain neurotransmitter that has been found by repeat studies to be elevated threefold in the spinal fluid of fibromyalgia patients. Two hormones that have been shown to be abnormal are cortisol and growth hormone. In addition, modern brain imaging techniques are being used to explore various aspects of brain function--while the structure may be intact, there is likely a dysregulation in the way the brain operates. The body's response to exercise, stress and simple alterations in position (vertical versus horizontal) are also being evaluated to determine if the autonomic nervous system is not working properly. Your body uses many neurotransmitters, such as norepinephrine and epinephrine, to regulate your heart, lungs and other vital organs that you don't have to consciously think about. Ironically, many of the drugs prescribed for FMS/CFS may have a favorable impact on these transmitters as well."