Adenomyosis may be involved by endometrial adenocarcinoma, but in contrast to true myometrial invasion, the depth of an adenomyotic focus involved by carcinoma does not alter pathologic tumor staging. Therefore, distinction from carcinoma invading myometrium is clinically relevant. We hypothesized that CD10, a marker of non-neoplastic and neoplastic endometrial stroma, would highlight the stromal component of adenomyotic foci and be useful in this distinction. Thirty-nine cases of endometrial adenocarcinoma were analyzed and divided into three groups: I, invasive endometrial adenocarcinoma (n = 14); II, endometrial adenocarcinoma involving adenomyosis but without myometrial invasion (n = 18); and III, adenomyosis involved by endometrial adenocarcinoma with concomitant invasive component (n = 7). All cases of adenomyosis involved by endometrial adenocarcinoma demonstrated CD10 expression in the stromal cells of adenomyotic foci. Eleven of 21 cases (52%) of invasive adenocarcinoma also showed CD10 expression, at least focally, in cells immediately surrounding the infiltrating glands. Of these, two cases (from Group III) also had associated adenomyotic involvement by carcinoma. The remaining cases of invasive carcinoma were negative for CD10. Therefore, presence of CD10 staining immediately surrounding neoplastic glands does not equate with involvement of adenomyosis by endometrial adenocarcinoma. In contrast, absence of CD10 expression excludes involvement of adenomyosis by adenocarcinoma.