A case of atypical polypoid adenomyoma (APA) of the uterus in an 81-year-old patient is reported. The tumor arose in the uterine fundus as a polypoid mass and measured 9 cm in its longest diameter. Its histological appearance was similar to that of the reported cases, i.e. presence of atypical endometrial glands and a cellular smooth muscle stroma. However, the patient's age was extremely advanced, since APA has predominantly been reported to arise in premenopausal women. APA has been suggested as being related to prolonged estrogenic stimulation. But the fact that in this case the uninvolved endometrium was absolutely atrophic, negated this possibility. Immunohistochemistry for alpha smooth muscle actin was positive diffusely in the stroma. The proliferative status of the neoplastic glands was assessed by immunostaining for Ki-67 antigen. About 30% of the glands contained Ki-67 positive nuclei, and about 3% of the nuclei were positive in the glands. These figures are far below those of the usual endometrial adenocarcinoma, strongly suggesting that the APA was benign in nature.