This report presents a detailed light-microscopic evaluation of biopsies obtained from 20 patients with eosinophilic fasciitis, a newly recognized disorder characterized by inflammation and thickening of the deep fascia, hypergammaglobulinemia, and peripheral and tissue eosinophilia. Early in the course of the disease, the deep fascia and lower subcutis are edematous and infiltrated with lymphocytes, plasma cells, histiocytes, and eosinophils; these features are associated with impressive peripheral eosinophilia. As the illness progresses, these structures and eventually the dermis become collagenized, thickened, and sclerotic. Tissue eosinophilia may be focal or diffuse and is usually observed in the fascia and/or lower subcutis. Extracutaneous involvement has been limited to a chronic synovitis and tenosynovitis, the latter frequently associated with the carpal tunnel syndrome. Deposits of immunoglobulin and/or complement were found in five of eight biopsies studied by direct immunofluorescence, which suggests that an immunologic stimulus may be responsible for initiating this syndrome. Differential diagnoses are discussed.