To our knowledge, familial occurrence of small-cell carcinoma of the ovary in first-degree relatives has not been described before. We studied two separate cases of small-cell carcinoma in a 21-year-old patient and in her 40-year-old mother, both of whom died of disseminated disease. Both tumors were studied with immunohistochemistry, using a variety of antibodies, and with flow cytometry. Microscopically, the tumor in the daughter was of the usual small-cell type, while that of her mother belonged to a large-cell variety. Immunohistochemically, both tumors showed some degree of epithelial differentiation but also expressed reactivity to some other antigens, most notably to smooth-muscle actin and muscle-specific actin. Both tumors showed a diploid DNA pattern and low S-phase fraction with flow cytometry. Neither of the cases expressed hypercalcemia. Small-cell carcinoma of the ovary may appear as a small-cell or large-cell variety in a familial setting. A diploid DNA pattern appears to be characteristic. Because this tumor was first reported relatively recently, the frequency of its familial occurrence is not known.