Three cases of endometrial stromal sarcoma (ESS) with prominent glandular differentiation within the primary or recurrent tumors are described. Each case posed a problem in diagnosis and classification. The patients, who ranged in age from 41 to 47 years, presented with abnormal uterine bleeding, a pelvic mass, or a combination thereof. All the patients underwent hysterectomy with or without bilateral salpingo-oophorectomy. There was no evidence of extrauterine spread of tumor in any case. Polypoid tumors involved the endometrium in two cases, and in one of them, tumor deeply invaded the myometrium. The tumor in the third case was an infiltrative mass that was confined to the myometrium and its vessels. On microscopic examination, the tumors were low-grade ESSs that contained large numbers of endometrioid glands, which were benign appearing in two cases, and in the third varied from atypical to carcinomatous. In one of the cases in the first group, the glandular component was present only in recurrent tumor excised 10 years after hysterectomy; prominent foci of sex-cord-like differentiation were also present in the recurrent tumor. This patient was clinically free of tumor 27 months later; follow-up in the other two patients was uneventful. ESSs with prominent numbers of benign-appearing glands should be distinguished from adenomyosis, endometriosis, ESSs arising in adenomyosis or endometriosis, ESSs with sex-cord-like differentiation, and müllerian adenosarcomas. ESSs with carcinomatous glands should be distinguished from endometrial adenocarcinomas and malignant müllerian mixed tumors (carcinosarcomas). Extrauterine lesions that have been designated "aggressive endometriosis" may be examples of extrauterine ESS with prominent glandular differentiation.