Adenocarcinomas arising from adenomyosis uteri are rare. This study reports four such cases and characterizes them clinically and microscopically. In all four patients, the endometrial cytology was negative, and MR imaging and ultrasound sonography did not detect the tumors preoperatively. The histological subtypes of the four tumors were endometrioid (one grade 1, one grade 3), serous, and clear cell. In three cases, the adenocarcinomas were present exclusively in the myometrium, and a transition between the carcinomas and the adenomyotic glands was observed in all cases. The eutopic endometrium was normal except in one case in which there was a small focus of invasive carcinoma. In two of four cases, pelvic or paraaortic lymph node metastases were present. In the carcinomas, ER immunoreactivity was not found in any tumor and PR positivity was found in only one tumor. In contrast, p53 immunopositivity was found in three of four carcinomas. Adenocarcinomas arising from adenomyosis are difficult to diagnose preoperatively, and their aggressive behavior in some cases seems to be related to the histological subtype.