The morphological features of epithelioid sarcoma may closely mimic those of epithelial neoplasms, such as squamous cell carcinoma, mesenchymal tumors, such as benign fibrous histiocytoma, and nonneoplastic lesions, such as granuloma annulare. Immunohistochemistry, particularly for epithelial markers and CD34, thus plays a valuable role in the differential diagnosis of epithelioid sarcoma. However, some epithelioid sarcomas may show very focal or even absent expression of such markers and may be difficult to distinguish from various morphological mimics. There is therefore continued interest in the development of new immunohistochemical markers of epithelioid sarcoma. Recently, loss of expression of INI1, a tumor suppressor gene/protein, and expression of GLUT-1, a glucose transporter protein, have been described in epithelioid sarcoma. We examined the utility of immunohistochemistry for INI1 and GLUT-1 in the diagnosis of epithelioid sarcoma and various cutaneous mimics. Twenty-four cases of epithelioid sarcoma, 13 cases of granuloma annulare, 10 cases of rheumatoid nodule, 19 cases of cutaneous squamous cell carcinoma, 7 cases of atypical fibroxanthoma, 9 cases of benign fibrous histiocytoma (dermatofibroma), and 3 cases of nodular fasciitis were immunostained for GLUT-1 and INI1 using commercially available antibodies, heat-induced epitope retrieval, and the Dako Envision detection system. Total or near-total loss of normal constitutive expression of INI1 protein was noted in more than 85% of epithelioid sarcomas, with 19 of 24 cases (79%) showing complete loss of INI1 expression. In contrast, all other cases studied showed uniformly retained expression of INI1. GLUT-1 was positive in 40%-50% of epithelioid sarcomas, all cases of granuloma annulare and rheumatoid nodules, 67% of benign fibrous histiocytomas, and in all squamous cell carcinomas. In contrast, atypical fibroxanthomas and cases of nodular fasciitis were consistently GLUT-1 negative. We conclude that immunohistochemistry for INI1 expression should be included as part of the routine immunohistochemical panel for the diagnosis of epithelioid sarcoma, along with established markers such as wide-spectrum cytokeratins, cytokeratin 5/6, p63, and CD34. In this clinical context, loss of INI1 expression seems to be an entirely specific marker of epithelioid sarcoma and this finding may be of great value in distinguishing CD34-negative epithelioid sarcoma from squamous cell carcinoma and in the distinction of rare cytokeratin-negative epithelioid sarcomas from necrobiotic processes, nodular fasciitis, and benign fibrous histiocytomas. In contrast, there does not seem to be a role for GLUT-1 immunohistochemistry in this differential diagnosis.