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Clinicopathologic features of 12 tumors of the lower uterine segment (isthmus) were studied and the findings compared with data on 196 tumors of the corpus endometrium proper. The mean age of these Japanese patients with tumors of the isthmus was 47 years and that of the patients with tumors of the corpus endometrium was 55 years. Seven of the isthmic tumors (58%) were graded as 3 (poorly differentiated) with histologic features of ad-enosquamous carcinoma, and all 12 tumors showed myometrial invasion. En-dometrial hyperplasia or atypical endometrial glands were found in the endometrium adjacent to the tumor in 11 women. Normal mucosal structures of the lower uterine segment were uninvolved, in the area between the tumor and the endocervix, in only three patients. Four patients died of the disease within 36 months after surgery. In a comparison with the usual endometrial tumors of the corpus, tumors of the lower uterine segment tended to be high-grade adeno-squamous carcinomas and myometrial invasion was more extensive. The short-term follow-up revealed that lower uterine segment tumors carried an unfavorable prognosis.