Comparison of ultrastructural features between leiomyosarcoma and leiomyoma of the duodenum was made. There was no absolute difference between the two, but the following ultrastructural features of leiomyosarcoma were different from leiomyoma: (1) occasional conglomerations of mitochondria surrounded by granular endoplasmic reticulum; (2) quite rare presence of single cilia; (3) frequent occurrence of intranuclear inclusion bodies; (4) variability in nuclear size and shape; and (5) wide interstitial space with scanty collagen fibrils. Although both tumors were considered to be of smooth muscle origin, their ultrastructural features were distinctively different from the normal smooth muscle in some points. In addition to the light microscopic findings, ultrastructural features were considered to be valuable for distinguishing the malignant from the benign smooth muscle tumors in the gastrointestinal tract.