The clinical and pathologic features of 28 Sertoli tumors of the ovary are presented. Seventeen of the 18 neoplasms had a functional effect. The effects were hyperestrinism (eight women), isosexual precocious puberty (three girls), virilization (four women), a progestational decidual reaction on the peritoneum (one woman), and a combination of virilizing and estrinizing signs (one woman). There were three histologic patterns: simple tubular, complex tubular, and folliculome lipidique. Two neoplasms recurred, both of which had the distinctive feature of invasion of the supporting stroma by individual and columns of tumor cells in the original tumors. Electron microscopic evaluation of three neoplasms disclosed. Charcot-Böttcher filaments unequivocally identifying the Sertoli differentiation of the predominant cells in the neoplasm. The identification by electron microscopy of a smaller population of cells within the three tumors similar to typical ovarian stromal cells suggests that Sertoli tumors arise from ovarian stromal cells.