Virilization caused by ovarian tumors with functioning stroma during pregnancy is extremely rare and has been reported in many ovarian tumors. In mucinous cystadenomas with maternal virilization during pregnancy the stromal cells responsible for the hormone secretion resemble lutein or Leydig cells and have been referred to as luteinized stromal cells. We present a case of a 30-year-old, gravida 2, para 1, woman who presented at approximately the 38th week of pregnancy with features of virilization. At the same time, a cesarean section was performed because of fetal distress and a male weighing 3,030 g without any gross abnormalities was deliv­ered. A large tumor of the right ovary was detected and a right salpingo-oophorectomy was performed. Histopathologically, the tumor proved to be a benign mucinous cystadenoma. Masses typically resembling lutein stromal cells or Leydig cells of the testes or ovarian hilus were found in the wall of the cyst below the mucinous epithelium. No crystalloids of Reinke were identified. The stromal component of the tumor was characterized as functioning stroma with luteinized stromal cells. The glandular mucinous epithelium showed focal positivity for human chorionic gonadotrophin. The cytoplasm of the luteinized stromal cells reacted strongly and diffusely with antiserum for vimentin. Also, the cytoplasm of the luteinized stromal cells showed focal intense positivity for synaptophysin, and focal mild positivity for human chorionic gonadotrophin. Staining results for oestrogen and proges­terone receptors were negative. In conclusion, we present an unusual case of clinical virilization during pregnancy associated with an ovarian mucinous cystadenoma with functioning stroma. The virilizing manifestations disappeared after removal of the ovarian neoplasm, supporting the perception that the functioning ovarian stroma was responsible for the androgen production.