Rationale:Anti-N-methyl-D-aspartate receptor (NMDAR) is the most common type of autoimmune encephalitis mediated by NMDAR antibodies. The pathological process remains unclear, especially in patients without tumors or infections. Autopsy and biopsy studies have rarely been reported because of the favorable prognosis. Pathological findings generally demonstrate mild-to-moderate inflammation. This case report presents severe anti-NMDAR encephalitis in a 43-year-old man without any identified triggers. The biopsy in this patient showed extensive inflammatory infiltration with evident B cell accumulation, which enriches the pathological study of male anti-NMDAR encephalitis patients without comorbidities.Patient concerns:A 43-year-old previously healthy man presented with new-onset seizures with recurrent jerks. The initial autoimmune antibody test with serum and cerebrospinal fluid yielded negative results. After ineffective treatment for viral encephalitis, based on the imaging results indicating the possibility of diffuse glioma, the patient underwent a brain biopsy in the right frontal lobe to rule out malignancy.Diagnosis:The immunohistochemical study showed extensive inflammatory cell infiltration, consistent with the pathological changes in encephalitis. Cerebrospinal fluid and serum samples were then retested and tested positive for IgG antibodies against NMDAR. Therefore, the patient was diagnosed with anti-NMDAR encephalitis.Interventions:The patient was administered intravenous immunoglobulin (0.4 g/kg/d for 5 days), intravenous methylprednisolone (1 g/d for 5 days, 500 mg/d for 5 days, subsequently reduced to oral administration), and intravenous cyclophosphamide cycles.Outcomes:The patient developed refractory epilepsy 6 weeks later and required mechanical ventilation. Despite brief clinical improvement after extensive immunotherapy, the patient died from bradycardia and circulation.Lessons:Anti-NMDAR encephalitis cannot be ruled out even if the initial autoantibody test result is negative. For progressive encephalitis of unknown etiology, it is necessary to recheck cerebrospinal fluid for anti-NMDAR antibodies.