Brenner tumor and struma ovarii, two uncommon ovarian tumors arising alone or together with dermoid cysts or adenomas, are both rare entities. Both tumors rarely become malignant and rarely metastasize. Few published reports describe coexisting Brenner tumor and malignant struma ovarii. Patients in whom these malignancies coexist only occasionally have peritoneal spreading, strumosis, or a history of thyrotoxicosis. The patient we describe, a 74-year-old woman, presented with a 2 months' history of lower abdominal pain and episodic intestinal subocclusion due to a complex pelvic mass. The mass consisted predominantly of a Brenner tumor associated with struma ovarii containing a single small island of thyroid tissue that had undergone malignant transformation into a well-differentiated papillary carcinoma and also normal thyroid tissue that had spread to the peritoneum. The patient underwent radical surgical treatment and after 7 years follow-up is disease free.