Looking at the results of the seven types of studies discussed previously, it appears that there is strong evidence for an association between fibromyalgia and major depressive disorder on the basis of (1) overlapping symptomatology, (2) similar pattern of comorbid disorders, and (3) high rates of major depressive disorder among relatives of patients with fibromyalgia. There is additional support for an association on the basis of responses to psychological tests and rating scales and the high lifetime rates of mood disorders in fibromyalgia. Two lines of evidence, (1) response to antidepressant medications and (2) response to biologic tests, offer little evidence either for or against an association. On balance, then the weight of the evidence favors an association between fibromyalgia and major depressive disorder. We therefore turn to an analysis of the nature of the association.