Adenomyosis is commonly seen in association with endometrial adenocarcinoma where it may or may not be involved by malignancy. This study of grade 1 endometrioid adenocarcinoma investigates whether patients with cancer-positive adenomyosis are at a different risk for deep myometrial invasion compared with those with cancer-negative adenomyosis. Ninety-three hysterectomy specimens with FIGO (International Federation of Gynecologists and Obstetricians) grade 1 endometrial endometrioid adenocarcinoma associated with adenomyosis were studied. Four experienced gynecologic pathologists retrospectively reviewed all hematoxylin and eosin-stained sections. Myometrial invasion was confirmed by CD10-negative staining around glands with jagged outline surrounded by inflamed desmoplastic stroma. Adenomyosis was involved by adenocarcinoma in 46 cases, whereas it was carcinoma-negative in 47 cases. Myometrial invasion was found in significantly more carcinoma-positive adenomyosis cases (n = 42, 91.3%) than with carcinoma-negative adenomyosis cases (n = 30, 63.8%) (chi(2) = 12.10; P = .0005). Moreover, myometrial invasion in the outer half was also seen in significantly more carcinoma-positive adenomyosis cases (n = 16, 34.8%) than with carcinoma-negative adenomyosis cases (n = 3, 6.4%) (chi(2) = 11.53; P = .0007). Among all cases of FIGO grade 1 endometrial endometrioid adenocarcinoma associated with adenomyosis, the ones that extend in the adenomyosis gain more invasive advantage, probably through increasing the surface area of its interface with the adjacent myometrium. When compared with tumors that do not involve adenomyosis, these tumors are not only more likely to invade the myometrium but are significantly more prone to achieve deep invasion into the outer half.