The clinicopathologic, immunocytochemical, and ultrastructural features of 15 small-cell undifferentiated carcinomas (SCUC) of the uterine cervix are reported. Patients ranged in age from 25 to 87 (median, 42 years) and presented as stages IB (nine patients), IIA (one patients), IIB (two patients), IIIB (two patients), and IV (one patient). A variety of treatment regimens were employed. Ten patients died of disease (3-71 months; median, 11 months), one patient has a suspicious lung nodule 10 months after diagnosis, one patient is comatose with brain metastases 4 months after diagnosis, and three patients are alive and well 5, 11, and 78 months after diagnosis. Histologically and cytologically, the tumors were identical to pulmonary small-cell undifferentiated carcinoma. Six tumors were associated with other forms of carcinoma, in situ or invasive or both, including epidermoid carcinoma in situ (one case), adenocarcinoma (three cases), and epidermoid carcinoma (three cases). All 13 tumors expressed one or more epithelial markers and at least one neuroendocrine marker. Electron microscopy demonstrated dense-core granules in six of seven tumors, dendrite-like processes in seven tumors, filament bundles in four tumors, and intracytoplasmic lumina in one tumor. Small-cell undifferentiated carcinoma of the cervix is an aggressive tumor with a propensity for rapid metastasis and high mortality. These tumors may demonstrate multidirectional differentiation, including the frequent expression of neuroendocrine features.