Eleven primary ovarian tumors that resembled small cell carcinoma of the lung are reported. They occurred in women 28-85 (mean 59) years of age, most of whom presented with abdominal swelling. Six of the tumors were unilateral and five bilateral; seven had spread beyond the ovary. The tumors ranged from 4.5 to 26 (mean 13.5) cm in greatest dimension and were mostly solid, with a variable minor cystic component. Microscopic examination showed small to medium-sized round to spindle-shaped cells with scanty cytoplasm, hyperchromatic nuclei, and inconspicuous nucleoli growing in sheets, closely packed nests, and occasionally islands and trabeculae. A component of endometrioid carcinoma was present in four tumors, another tumor showed squamous differentiation, another contained a cyst lined by atypical mucinous cells, and two others were associated with a Brenner tumor. Argyrophil granules were present focally in two of six tumors appropriately stained. Immunohistochemical staining was performed in nine cases: in six there was staining of the small cell component for keratin, in five for epithelial membrane antigen, in seven for neuron-specific enolase, in two for chromogranin, and in one for Leu-7. Vimentin staining was not observed. Flow cytometry was performed on eight tumors: five were aneuploid and three diploid. Five of seven patients with long-term follow-up died of, or with, disease at 1-13 (mean 8) months, one died after an unknown interval, and one was alive at 7.5 years. Two other patients had recurrent or residual disease at 6 and 8 months.