ABSTRACT During deployment, a 52-year-old male developed acute behavioral changes. Though initially considered to have PTSD and related agitation and confusional state, his evaluation was consistent with acute encephalopathy. Magnetic resonance imaging of the brain showed T2 hyperintensities, and CSF analysis was positive for anti-N-methyl-D-aspartate receptor antibody. A nuclear protein in testis carcinoma midline carcinoma was discovered in the lung. Immunotherapy and surgical resection led to steady improvement prior to adjuvant chemotherapy. Autoimmune encephalitis due to anti-N-methyl-D-aspartate receptor antibodies is increasingly being recognized as causal of acute behavioral change.