Six cases of locally aggressive and/or potentially malignant glomus tumors are described. On the basis of clinical and pathologic criteria, the following classification is proposed. The first category is a locally infiltrative glomus tumor (LIGT) which has the usual glomus histologic features. The second group is a cytologically malignant tumor arising and merging with a typical glomus tumor, designated glomangiosarcoma arising in a benign glomus (GABG). The third category and the most difficult to recognize is the de novo glomangiosarcoma (GADN), which must be distinguished from other round cell sarcomas. Most of these locally aggressive glomus tumors are vimentin positive and are immunoreactive for muscle-specific actin. Electron microscopic examination in one GABG case showed cells with numerous microfilaments and pinocytotic vesicles; a second GADN case contained cells with microfilaments and an incomplete basal lamina. As a group these locally aggressive or potentially malignant glomus tumors are larger and more deeply located than the conventional glomus tumor. Although 50% of these tumors recurred locally, none have metastasized.