Two different diseases may cause the same syndrome. Two different syndromes may be caused by the same disease. Although CFS and FMS are differently defined syndromes, it is likely that patients with fibromyalgia and chronic fatigue syndrome have the same underlying disease for a number of reasons. Three quarters of patients who meet the criteria for each do so for the other. It seems unlikely that these patients have two separate diseases.
Symptoms are similar in the two disorders.
Effective treatments for one often turn out to be effective for the other. In FMS as in CFS, too much or too vigorous exercise is harmful. Similar laboratory abnormalities are now being uncovered in both. This has lead to the suggestion that the distinction between the two is artificial and exists largely for historical reasons. Perhaps dividing the two disorders on the basis of degree of fatigue relative to pain is no less arbitrary than doing so on the basis on the degree of dizziness, tingling, pelvic pain, or any of the other dozen or more symptoms associated with these disorders.