Two cases of aggressive fibromatosis of the thyroid gland region were studied by histological, immunhistological and electron-microscopic methods. Contractile cells can be found in all tissue preparations using human serum containing antibodies to smooth muscle, especially in the periphery of the tumor. These cells have the characteristics of myofibroblasts by electron microscopy. The central areas of the tumor are composed of hyalin thickened bundles of collagen fibres with few cellular elements. The myofibroblast is thus considered to be the characteristic cell type of the tumor, and the pathogenesis and histogenesis of the lesion are discussed.