Background. Although mature cystic teratoma is the most common tumor of the ovary, squamous cell carcinoma arising from a mature teratoma is a rare event. Prognosis depends on clinical stage, grade, and recently described mode of tumor infiltration. Case. This case involves a 52-year-old woman with stage II squamous cell carcinoma arising in a mature cystic teratoma of the left ovary. Final pathology demonstrated poorly differentiated squamous cell carcinoma with gamma mode of tumor infiltration. The patient completed adjuvant chemotherapy and subsequently died of progressive disease. Conclusion. Squamous cell carcinoma arising from a benign cystic teratoma is a rare event. Studies have shown stage, grade, and mode of infiltration to be predictors of recurrence and prognosis. This case supports the growing evidence linking the mode of tumor infiltration with overall prognosis of survival. © 2005 Elsevier Inc. All rights reserved.