Lymphovascular invasion is an adverse prognostic factor in breast cancer. The lymphatic endothelial marker D2-40 has been shown to improve accuracy in detecting lymphovascular invasion. In addition to marking lymphatic endothelium, D2-40 has been cursorily noted to react with breast myoepithelium. The extent of this expression and the potential for misinterpreting in situ carcinoma as lymphovascular invasion because of D2-40-positive myoepithelium have not been formally addressed. The aim of this study was to determine the scope of breast myoepithelial expression of D2-40 and to identify problematic patterns of expression by in situ carcinoma that could be confused with lymphovascular invasion. We evaluated the distribution and intensity of D2-40 immunohistochemical expression in breast myoepithelium in normal breast (n = 50), proliferative fibrocystic changes (n = 10), ductal carcinoma in situ (n = 35), and lobular carcinoma in situ (n = 5). All cases of normal breast exhibited a variable degree of D2-40 expression by myoepithelium. The distribution was patchy and the intensity was less than that of the adjacent lymphatic endothelium. Larger ducts were more often positive than terminal ducts and lobules. D2-40 marked 77% of ductal carcinoma in situ cases and all lobular carcinoma in situ cases to a variable degree. Only a minority of involved ducts were reactive in each positive case; the intensity was weak to moderate. Although the tumor growth pattern generally enabled distinction of ductal carcinoma in situ from lymphovascular invasion, D2-40 myoepithelial expression in small ducts completely filled by solid-pattern ductal carcinoma in situ mimicked the pattern expected for lymphovascular invasion. Morphology of these myoepithelial cells was not diagnostically helpful as their compressed, stretched-out shape mimicked that of endothelium. Myoepithelial markers (p63 and smooth muscle myosin) confirmed each diagnosis of ductal carcinoma in situ. Lobular carcinoma in situ posed similar problems. The interpretation of D2-40 in the breast requires awareness that myoepithelium may also be immunoreactive. Solid-pattern ductal carcinoma in situ and lobular carcinoma in situ may be misinterpreted as lymphovascular invasion. We recommend that myoepithelial markers be used in conjunction with D2-40 to distinguish solid intralymphatic tumor emboli from solid pattern in situ carcinoma.