Fibrositis is considered primary when there is no associated underlying disorder, and secondary when it occurs in patients with underlying rheumatic or other organic disease. Since fibrositis has become better defined, the list of underlying disorders has grown, and its identification requires careful diagnostic study. The differentiation of primary and secondary fibrositis has therapeutic implications. Secondary fibrositis should respond to treatment of the underlying disorder, whereas the primary syndrome may require additional management directed to the musculoskeletal pain and sleep and emotional disturbances commonly recognized as major manifestations of this syndrome.