The clinicopathologic and immunohistochemical findings in 30 cases of idiopathic fibroinflammatory lesions of the mediastinum are presented. There were 17 male and 13 female patients between 10 and 64 years of age; 19 were African-American, and 10 were Caucasian. Clinically, respiratory and/or systemic symptoms including cough, shortness of breath, and fever were present in 28 patients. Five patients also presented with evidence of superior vena cava syndrome. All of the lesions involved the anterior mediastinum with radiographic evidence of hilar and paratracheal involvement in nine and five patients, respectively. Histologically, the lesions were characterized by an inflammatory fibrosing process that showed three distinctive histologic patterns. On the basis of the histologic pattern, they were subdivided into three distinct groups (stages). Stage I demonstrated edematous fibromyxoid tissue with numerous spindle cells, eosinophils, mast cells, lymphocytes, plasma cells, and thin-walled blood vessels; Stage II showed thick glassy bands of haphazardly arranged collagen with focal interstitial spindle cells, lymphocytes, and plasma cells; and Stage III was characterized by dense acellular collagen with scattered lymphoid follicles and occasional dystrophic calcification. Immunohistochemical studies in 17 cases highlighted large numbers of vimentin- and actin-positive spindle cells and capillary-like vessels in Stage I lesions, with fewer numbers of vimentin-positive, actin-negative spindle cells and vessels in Stage II lesions. Our findings suggest that "sclerosing mediastinitis" represents the final stage of an evolving, dynamic process with different morphologic appearances akin to abnormal wound healing. Thus, we propose the term fibroinflammatory lesion of the mediastinum to convey the true nature of the process.