The clinicopathologic and immunohistochemical findings of an unusual case of ovarian yolk sac tumor associated with endometrioid carcinoma and mucinous cystadenoma of the ovary are reported. The tumor was detected in a 51-year-old postmenopausal woman who presented with abdominal swelling and a pelvic mass. The patient underwent total abdominal hysterectomy with bilateral salpingo-oophorectomy and omentectomy. The tumor was confined to the right ovary and measured 16.0 cm in greatest dimension. Microscopic examination revealed that the tumor had a yolk sac component associated with an endometrioid carcinoma, grade I, and a mucinous cystadenoma. A background of atypical endometriosis was noted. Immunoperoxidase studies showed that the yolk sac component stained diffusely with a cytokeratin cocktail and was focally positive for alpha-fetoprotein. It was negative for keratin 7. In contrast, the endometrioid carcinoma stained positive for keratin 7 in addition to the cytokeratin cocktail, but was negative for alpha-fetoprotein. After surgery, the patient received three cycles of chemotherapy. However, the disease progressed and the patient died 10 months after the diagnosis of the ovarian tumor.