Ultrastructural data about large cell variant ovarian small cell carcinoma (LCV-SCC) are scarce and contradictory and the role of transmission electronmicroscopy (TEM) is not clear in the assessment of such tumors. The authors present a case of LCV-SCC without hypercalcemia in a 30-year-old woman. The diagnosis was confirmed by histopathological and immunohistochemical studies. Cytopathological examination of peritoneal washing showed a population of large neoplastic cells. TEM demonstrated that the neoplasia comprised two types of cells: one type showed many coarse secretory granules without dense core, and the other type was without granules and showed dilated endoplasmic reticulum and sometimes indented nuclei. The present case indicates that different underlying ultrastructural patterns, not yet well known, exist in connection with the pathological and clinical behaviour of LCV-SCC. TEM might play a role in the identification of subtypes of LCV-SCC with different prognostic and therapeutic impact.