We describe an unusual ovarian neoplasm in a 57-year-old woman composed of an admixture of mucinous cystadenoma and adult granulosa cell tumor (AGCT). In areas the two components were separate but elsewhere there was intermingling of the two elements. The combination of mucinous cystadenoma and AGCT has only rarely been reported. Theories of histogenesis include a collision tumor and heterologous mucinous differentiation within an AGCT. We favor the latter theory in this case, because in many areas there was an intimate admixture of the two components. Because heterologous mucinous elements are well described in other ovarian sex-cord-stromal neoplasms, especially but not exclusively Sertoli Leydig cell tumors, it is not unexpected that a similar phenomenon could occur in an AGCT. We review the previously reported cases of combined mucinous cystadenoma and granulosa cell tumor of the ovary.