Sixteen cases of small-cell carcinoma of the endometrium were encountered in patients who ranged in age from 30 to 78 (mean, 57.4) years. Of the 12 patients whose presenting features are known, eight had abnormal vaginal bleeding, three had pain related to metastatic tumor, and one patient had both symptoms. On pelvic examination, adnexal masses were palpable in three patients, and vaginal involvement was evident in two; one patient had a large palpable periumbilical mass. Thirteen patients underwent total abdominal hysterectomy and bilateral salpingo-oophorectomy. Extrauterine spread was documented intraoperatively in eight cases, including widespread intraabdominal and ovarian metastases in four cases, vaginal involvement in the two cases noted previously, paraaortic lymph node involvement in one case, and tubal involvement in one case. Three tumors were International Federation of Gynecology and Obstetrics (FIGO) stage I, four were stage II, two were stage III, and six were stage IV; in one case, there was insufficient information to allow staging. On gross examination, the tumors were usually described as bulky, ill-defined, and invasive of the myometrium; four were polypoid. Microscopic examination revealed sheets, cords, and nests of small or intermediate-sized cells with scanty cytoplasm, hyperchromatic nuclei, and a high mitotic rate. Single-cell and zonal necrosis and vascular invasion were typically present. Synchronous grade 1 or grade 2 endometrial endometrioid adenocarcinoma was present in eight cases, and complex atypical endometrial hyperplasia, in two others. In three cases, the adenocarcinoma merged almost imperceptibly with the small-cell component. None of the tumors contained argyrophil or argentaffin cells, although nine of 11 tumors were immunoreactive for neuron-specific enolase (one of these was also Leu-7 positive), and another was chromogranin positive. Of the 11 cases with follow-up information, seven patients died of disease (at least four with distant metastases) with a median survival of 12 months, and another patient was alive with distant metastases at 18 months. The remaining patients were clinically free of disease at postoperative intervals of < or = 1 year (two cases) and 4.5 years (one case). This study confirms that small-cell carcinomas of the endometrium are a histologically distinctive subtype of endometrial carcinoma, which, like their counterparts in the uterine cervix, are aggressive tumors with a propensity for systemic spread and a poor prognosis.