The clinical picture, the light and electron microscopic appearance, and the histochemical findings are described in six cases of fibroma of tendon sheath. The ages at onset for the four men and two women range between 16 and 54 years. The lesions were well circumscribed, rounded or oval, and had a diameter of about 1 cm. Light-microscopically the lesions consisted of a dense fibrocollagenous tissue with a variable degree of collagenization and cellularity, the peripheral zone often being most cellular and containing slit-like vascular spaces. Electron-microscopically two main cell types were identified: myofibroblasts and fibroblasts, the former dominating in cellular areas, the latter dominating in collagenized areas. The histochemical analysis indicated the presence of sulphated glucosaminoglycans within the collagenous matrix. Immunoperoxidase staining for factor VIII RAG-related antigen gave prominence to the endothelium of capillary-like blood vessels. The differential diagnosis against giant cell tumor of tendon sheath is discussed. The observations indicate that fibroma of tendon sheath is a distinct entity of probable reactive, non-neoplastic nature.