Originally described as "fibrositis," fibromyalgia has long been considered a muscle disorder, and many studies have investigated the possible pathologic basis of the disorder by examining muscle tissue, using various methodologic approaches. Although initial studies suggested a possible pathologic basis in muscle, most had serious methodologic limitations. More recent studies, however, have avoided methodologic pitfalls and indicate that the muscles of patients with fibromyalgia are normal. When data from studies of tenderness are also taken into account, the weight of evidence suggests that fibromyalgia is a chronic pain syndrome which has a central rather than peripheral or muscular basis.