A case of glomangiosarcoma (GS) arising on the lateral side of the left upper arm is reported here. GS is very rare compared with benign glomus tumor (GT) and there has been some controversy regarding whether it is a true malignant tumor. Histologically, the tumor consisted of two different components. In the periphery of the tumor, a solid sheet-like growth of small cuboidal cells identical to benign GT was seen. However, in the central portion, the tumor was composed of short spindle cells with hyperchromatic atypical nuclei arranged in weak fascicles, mimicking leiomyosarcoma. Mitotic figures were not uncommon in the latter area. Immunohistochemistry revealed that both cuboidal and spindle-shaped tumor cells were positive for vimentin and muscle actin as were cells of other benign GTs used for controls. In addition, only spindle-shaped cells expressed protein produced by the suppressive oncogene p53 in the nuclei, a phenomenon not seen in the five benign GTs or in cuboidal cells of GS. These results suggest that GS may exhibit more aggressive behavior than benign GT.