The case of a 38-year-old woman with an ovarian mucinous cyst that contained solid nodules of anaplastic carcinoma is described. Hitherto, only 17 cases of this aggressive type of ovarian tumor have been reported. The patient underwent a total abdominal hysterectomy with bilateral salpingo-oophorectomy and a para-aortic and pelvic lymphadenectomy. The tumor was classified as FIGO Stage Ic. On flow cytometry the tumor was DNA diploid. Also, on image analysis the regions with well- differentiated carcinoma and the regions with anaplastic nodules were DNA diploid. No adjuvant treatment was given. There is no evidence of disease 30 months after diagnosis.