Of 31 patients with malignant teratomas, the average age of the 19 with solid tumors was 37.6 years and of the 12 with cystic tumors, 43 years. Among the patients over 20 years old, the incidence of nulliparity was 33 and 36% in the solid and cystic groups, respectively.The prognosis is universally poor except in cases of malignant struma ovarii or in instances in which the malignant element of the tumor consists solely of histologically demonstrated malignant neurogenic elements. The treatment of malignant struma ovarii is total abdominal hysterectomy, bilateral salpingo-oophorectomy, thyroidectomy, and I131. In all other instances of malignant teratoma, we think that total abdominal hysterectomy, bilateral oophorectomy, and external pelvic or abdominal irradiation is indicated.