Objective: Anti-NMDA receptor encephalitis is a paraneoplastic syndrome characterized by neuropsychiatric symptoms, involuntary movements, autonomic instability, and seizures. It is more common in young females and associated with a mature ovarian teratoma. We describe a classic presentation of a young female with mature ovarian teratoma, who had negative imaging on initial investigation for an ovarian neoplasm. Background A 25 year old female presented with seizure episode, headache, fever, and malaise. An extensive medical evaluation and review of her past medical and psychiatric histories were unremarkable. After admission she developed bizarre behaviors, hallucinations, and oral-facial dyskinesias, followed by dysautonomia resulting in transfer to ICU. Soon after, she was intubated for respiratory failure. Results: A lumbar puncture showed lymphocytic pleocytosis, with mildly elevated protein, and normal glucose. Due to concern for Anti-NMDAR encephalitis, a whole body CT and PET scan were obtained and reported unremarkable. Ultrasound and MRI pelvis revealed a “benign” right ovarian cyst. Gynecology was consulted; however, they felt an exploratory laparotomy was not warranted for a benign cyst. Six weeks following admission, Anti-NMDA receptor antibodies were reported positive. A repeat MRI pelvis, with fat saturation and in-out-of phase sequence, showed the right ovarian cyst consistent with a mature ovarian teratoma. She underwent bilateral salpingo-oophorectomy followed by five days of solumedrol, and cytoxan and rituximab a week later. Her mental state and involuntary movements continued to improve and was discharged to rehabilitation. Conclusions: Though, ultrasound is commonly used, MRI is more sensitive and specific for evaluation of adnexal masses. For ovarian teratomas, fat saturation along with in-and-out of phase sequence, should be included to detect the sebaceous content. If all imaging modalities continue to be negative, serial imaging is recommended. If clinical suspicion is high in a young female, exploratory laparoscopy or laparotomy should be considered for an ovarian teratoma. Disclosure: Dr. Ashraf has nothing to disclose. Dr. Janis has nothing to disclose. Dr. Walter has received personal compensation for activities with Deringer-Ney, Inc, Medtronic, Inc, Boehringer Ingelheim Pharmaceuticals and Teva Neuroscience as speaker, consultant and/or participant on an advisory board.