A previously healthy 14-year-old girl presented with a 1-year history of abdominal pain that had worsened during the past 4 days. She had a right lower abdominal mass that was initially diagnosed as an ovarian tumor. MR imaging revealed a unilaterally enlarged and partially torted left polycystic ovary. Polycystic ovary is a common cause of increased ovarian volume in women of reproductive age. It is characterized by numerous small peripherally located follicles and increased stroma. It may mimic a neoplasm and lead to difficulties in diagnosis. In this case report, we discuss the unusual MR imaging findings and the pitfalls in diagnosis.