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Small cell tumors of the ovary are uncommon but represent an important group to recognize in the differential diagnosis of primary and metastatic ovarian neoplasms. In some cases the correct diagnosis cannot be confidently made on the basis of clinical setting, routine light microscopy, and immunohistochemistry, and electron microscopy may be supportive or definitive in establishing cell type. The cell type is often important in choosing optimal therapy and in predicting prognosis. The authors performed electron microscopy on a moderate number of ovarian small cell tumors and here describe and illustrate the diagnostic features of representative examples of various types. The ultrastructural features of the metastatic tumors, such as embryonal rhabdomyosarcoma, neuroblastoma, and melanoma, are identical to those of their respective primary tumors, are well known, and usually pose no problem in diagnosis. On the other hand, the ultrastructural features of some primary ovarian small cell tumors may present a more difficult differential diagnosis, because they have features that are subtle and/or in common. Exemplary of tumors in this category are diffuse adult granulosa cell tumor, endometrial stromal sarcoma, and small cell carcinomas of the hypercalcemic and pulmonary (oat cell) types. Distinguishing among them may be difficult but is possible, and electron microscopy may be a valuable supplement to the diagnostic information obtained from the clinical presentation, light microscopy, immunohistochemistry and, in some tumors, cytometric analysis of these neoplasms.