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Anti-N-methyl D-aspartate receptor (NMDAR) encephalitis is a recently described severe neurological disorder predominantly affecting young women, which presents with psychosis, memory deficits, seizures, and encephalopathy, often requiring prolonged hospitalization. The condition is frequently associated with an underlying neoplasm, most often an ovarian teratoma, and in such cases appears to be a para-neoplastic, immune-mediated encephalopathy. The histologic features of the teratomas associated with anti-NMDAR encephalitis have seldom been described in detail. Therefore, in this report, we have compared ovarian teratomas (4 mature and 1 immature) from 5 patients with anti-NMDAR encephalitis with 22 sporadic control teratomas (14 mature and 8 immature) that included neuroglial elements. The encephalitis-associated tumors ranged from 0.7 to 9.5 cm diameter, and 1 case was bilateral; the second teratoma was discovered 13 mo after the first when symptoms recurred. In comparison with control teratomas, the anti-NMDAR-associated tumors showed a more marked intratumoral lymphoid infiltrate that colocalized to the mature neuroglial elements. Reactive germinal centers (3 cases) and diffuse lymphoplasmacytic infiltrates within the neuroglial matrix (4 cases), and degenerative neuronal changes (2 cases), were seen only in the anti-NMDAR-positive cases. Pathologists encountering ovarian teratomas with these distinctive reactive lymphoid elements should consider the possibility of anti-NMDAR encephalitis, particularly because the neurological symptoms may develop after tumor resection. Careful histopathologic examination may be required to identify small, radiologically occult teratomas, and to demonstrate the presence of subtle neoplastic neuroglial components in teratomas associated with anti-NMDAR encephalitis.