BACKGROUND Anti-NMDA-R receptor encephalitis occurs predominantly in younger women and is often comorbid with ovarian teratoma, a feature that is often absent in children. Here, we report our experience with two pediatric patients, in whom no tumors were present during treatment for encephalitis, but in whom ovarian teratomas developed without encephalitis relapse after treatment was completed. METHODS Patient 1 was a 14-year-old girl who was diagnosed due to characteristic symptoms and anti-NMDA-R antibody. MRI scanning during treatment revealed no ovarian tumors, but a tumor developed in the right ovary 10months after onset. Another tumor developed in the left ovary 3years after onset, and a mature ovarian teratoma was confirmed after bilateral partial ovariectomy. Patient 2 was an 11-year old girl who was also diagnosed due to characteristic symptoms and anti-NMDA-R antibody. Imaging during treatment revealed no ovarian tumors, but a 2.5-cm tumor mass was found in the left ovary 10months after onset, and a mature ovarian teratoma was confirmed after partial ovariectomy. CONCLUSIONS This case report suggests the need for regular tumor screening after treatment for anti-NMDA receptor encephalitis because of potential subsequent tumor development, even in pediatric patients who initially present with no comorbid tumors. No analysis of relapse risk has yet been reported in cases of tumor development after treatment, and at this point, whether or not resection is needed to prevent relapse remains unclear. However, because teratomas usually grow, have an associated risk of torsion, and can be malignant, tumor removal should be considered.