Adenomyosis is commonly seen in hysterectomy specimens for endometrial adenocarcinoma where it could be involved with the tumor. When adenocarcinoma involves adenomyosis, the tumor may remain limited to the adenomyosis or proceeds to invade the adjacent myometrium. The purpose of this study was to investigate whether the risk of myometrial invasion by grade 1 endometrioid adenocarcinoma in cases with cancer-positive adenomyosis is different from that of cases where cancer occurs in the absence of adenomyosis. Forty-six consecutive hysterectomy specimens with International Federation of Gynecology and Obstetrics (FIGO) grade 1 endometrial endometrioid adenocarcinoma involving adenomyosis and 49 consecutive specimens with the same tumor occurring in the absence of adenomyosis were retrospectively studied by 4 experienced gynecologic pathologists. In cases with adenomyosis, myometrial invasion was confirmed by CD10-negative staining around glands with irregular outline surrounded by inflamed desmoplastic stroma. Myometrial invasion was found in significantly more adenomyosis cases (n = 42, 91.3%) than in cases without adenomyosis (n = 38, 77.5%) (chi = 4.79, P = 0.03). In 16 cases of the former group, the invasion only occurred from the foci of adenomyosis. Although myometrial invasion in the outer half was more common in the adenomyosis group (n = 16, 34.8%) than in cases without adenomyosis (n = 9, 18.4%), the difference was not statistically significant (chi = 3.29, P = 0.07). By involving coexistent adenomyosis, FIGO grade 1 endometrial endometrioid adenocarcinoma is associated with myometrial invasion, probably through increasing the surface area of its interface with the adjacent myometrium. When compared with their counterparts that occur in the absence of adenomyosis, these tumors are significantly more likely to invade the myometrium.