A composite mucinous and granulosa cell tumor of the ovary in a 76-year-old woman is herein reported. At laparotomy this tumor proved to be a solid and cystic mass measuring 10 cm in greatest diameter. Many of the cysts were lined with a benign mucinous epithelium of the endocervical type, and solid areas contained a proliferation of granulosa cells. These two disparate components were intimately mixed. A theca cell component was also present in areas adjacent to the mucinous epithelium. The coexistence of mucinous and granulosa cell tumor is extremely rare and only four such cases have previously been reported in the literature, and the histogenesis of this tumor has not yet been elucidated. In the present case it is suggested that the granulosa cell element commenced as a reactive stromal hyperplasia in the wall of the pre-existing mucinous neoplasm and thereafter progressed to the point of producing a tumor-like mass or neoplastic changes.