We present an extensive immunohistochemical analysis of 7 mammary sarcomas that did not fit into any specific soft tissue sarcoma category. Histologically, they were composed of spindle cells with highly pleomorphic nuclei and abundant mitoses. Our immunohistochemical antibody panel included pan-cytokeratin (CK), basal cell type CKs (34betaE12, CK5/6, CK14, CK17) and vimentin antibodies, antibodies to established (SMA, CD10, p63, S-100, maspin, calponin, GFAP, SM-myosin), and novel (CD29, 14-3-3sigma) myoepithelial markers, as well as antibodies to CD34, desmin, h-caldesmon, steroid receptors (estrogen, progesterone, androgen), and EGFR (Her-1). Whereas CKs, CD34, desmin, and h-caldesmon were not expressed, all tumors were positive for CD10 and vimentin. CD29 and SMA were observed in 3 cases each (43%), and p63 and calponin in 2 cases each (29%). Other myoepithelial markers and steroid receptors were absent, except androgen receptors, which were expressed in one sarcoma. Five sarcomas showed positivity for EGFR. The distinction of specific, histogenetically defined sarcoma entities (such as leiomyosarcoma, angiosarcoma, liposarcoma) from NOS-type sarcoma with CD10 expression is usually clear-cut because the former exhibit a characteristic histomorphology and immunoprofile. Phyllodes tumors with stromal overgrowth or recurrent phyllodes tumors lacking epithelial structures as well as periductal stromal sarcomas can be ruled out by their frequent expression of CD34 and negativity for myoepithelial markers. The most important differential diagnosis is sarcomatoid metaplastic carcinoma because its treatment includes axillary lymphadenectomy. Since some NOS-type sarcomas with CD10 expression and most metaplastic carcinomas show positivity for CD29, SMA, and p63, differential diagnosis can be extremely difficult and requires extensive immunohistochemical evaluation for CKs and additional myoepithelial markers such as S-100, 14-3-3sigma, and maspin. The immunophenotype of NOS-type sarcomas with CD10 expression suggests that these neoplasms represent a mammary sarcoma variant with myoepithelial features.