<p>Anti-IgLON5 disease is a rare neuronal surface antibody-associated encephalopathy characterized by complex and diverse clinical manifestations, including sleep disturbances, bulbar symptoms, abnormal movements, cognitive decline, and occasionally, neuropsychiatric abnormalities. Immunotherapy is a mainstay of treatment. However, most patients still experience poor outcomes. We report the case of a 72-year-old female who presented with intractable insomnia and progressive mutism. Upon further evaluation, anti-IgLON5 antibodies were detected in both serum and cerebrospinal fluid via cell-based assay, leading to the final diagnosis of anti-IgLON5 disease. The patient failed to improve with intravenous immunoglobulin at the outside hospital; similarly, high-dose intravenous methylprednisolone at our institution yielded no clear benefit. Subsequently, efgartigimod, a neonatal Fc receptor inhibitor, was incorporated into the treatment regimen, resulting in significant clinical improvement, as evidenced by substantial reductions in both the modified Rankin Scale score and anti-IgLON5 disease composite score. To our knowledge, this is the first case report on efgartigimod for treating anti-IgLON5 disease. As an adjuvant therapy, efgartigimod demonstrated promising efficacy, suggesting its potential as a treatment option, especially in patients with limited response to conventional immunotherapy.</p>