Depth of myometrial invasion is accepted as one of the most important prognostic factors in carcinoma of the endometrium. In an effort to define the significance of adenomyosis containing adenocarcinoma as it relates to myometrial invasion, 52 cases of Stage I adenocarcinoma with coexisting adenomyosis were identified. A subset of 11 cases was noted to have adenocarcinoma in adenomyosis, invasive to a depth greater than would have been appreciated had this entity not been identified. These 11 cases had a 100% 5-year survival, suggesting that adenocarcinoma in adenomyosis does not indicate a more ominous prognosis. The depth of invasion into the myometrium proper is the significant prognostic factor. The pathologist must be able to differentiate these two distinct entities to accurately assess prognosis.