Twenty-seven atypical polypoid adenomyomas of the uterus were reviewed. The tumors occurred in women ages 21-53 (average 39.7) years old, who presented with abnormal uterine bleeding in most cases. Two of the patients had Turner's syndrome. The adenomyomas most frequently involved the lower uterine segment, measured up to 6 (average 1.9) cm in greatest dimension, and were characteristically polypoid and well circumscribed. Microscopic examination disclosed endometrial glands with varying degrees of hyperplasia and atypicality up to the appearance of carcinoma in situ within neoplastic smooth muscle. Squamous differentiation was common within the glands; the smooth muscle occasionally was atypical. Follow-up data suggested that the lesion is benign, although it persisted in two patients treated by curettage alone. Atypical polypoid adenomyomas should be distinguished from endometrial carcinomas and other malignant uterine neoplasms with which they are often confused, particularly in curettage specimens.