Soft-tissue tumor with abnormal amianthoid collagen fibers. Electron microscopic examination of a solitary soft-tissue tumor from the face demonstrated large areas with abnormal amianthoid collagen fibers in the neoplasm. The lesion was classified as a benign neoplasm of myofibroblasts and tentatively named a myofibroblastoma. The significance of the amianthoid collagen fibers is unknown. Normal native collagen fibers were found in some parts of the neoplasm. 