Patients with low-stage, low-grade endometrial adenocarcinomas have a favorable prognosis; however, a subset has a risk of recurrence and death. We were interested in evaluating patterns of myometrial invasion and correlating them with clinical outcome to potentially identify patients at increased risk. A total of 324 cases of low-stage Grade 1 endometrial adenocarcinoma were reviewed to identify those with myoinvasion. The myoinvasive cases were classified on the basis of the pattern of invasion: infiltrating glands, microcystic elongated and fragmented (MELF; a distinctive histologic variant of the infiltrative gland pattern), broad front, adenomyosis like, and adenoma malignum. Depth of invasion and lymphovascular invasion were recorded, and a clinical follow-up of at least 2 y was obtained, as most recurrences occur in this time frame. Ninety-eight of 324 (30%) cases were invasive; 75 had >2 y of follow-up, with an average length of follow-up of >7 y (range, 24-154 mo; mean 87 mo). All patients had a hysterectomy and bilateral salpingo-oophorectomy; 39 (52%) also underwent a lymphadenectomy. Twenty-seven (36%) were superficially invasive (<10% myoinvasion), 42 (56%) invaded 10% to 49%, and 6 (8%) invaded >50%. Six (8%) cases exhibited cervical stromal invasion (Stage II); the rest were Stage I (65 IA, 4 IB). The invasive patterns consisted of infiltrative glands (48; 65%), a broad front (16; 21%), MELF (5; 7%), adenomyosis like (5; 7%), and adenoma malignum like (1, 1%). There were 65 Stage 1A cases and, of these, the myoinvasive pattern was as follows: 41 infiltrating glands, 15 broad front, 5 MELF, and 4 adenomyosis like. There were 4 Stage IB cases, of which 2 had infiltrating glands, 1 had adenoma malignum, and 1 displayed adenomyosis-like invasion. Six (8%) cases had cervical stromal invasion (Stage II), of which 5 had an infiltrative pattern of growth and 1 displayed a broad front. Lymphovascular invasion was noted in 6 cases (8%), all of which had infiltrative glands. The majority of Grade 1 endometrioid endometrial adenocarcinomas do not invade the myometrium. In cases with invasion, the infiltrative gland pattern was associated with higher stage, (3/4 Stage IB, 5/6 Stage II), lymphovascular invasion (4/6 cases), and recurrence (2/75 cases), suggesting that this growth pattern may be associated with tumors having other histologic features typically associated with more aggressive behavior.