Malignant change to glioblastoma multiforme in a mature cystic teratoma (dermoid cyst) is exceptionally rare. Besides a report on such a case we give a brief review of the literature on this subject and a comment on treatment. The reported case is about a 41-year-old woman with a mature cystic teratoma of the ovary with transformation to glioblastoma multiform. The tumor was limited to the ovary and completely excised by salpingo-oophorectomy. Three and a half years after surgery the patient is alive without evidence of recurrent disease. For limited stage Ia tumors we found support in the literature for expectant management after surgery, without adjuvant chemotherapy.