BACKGROUND Despite the preponderance of evidence of immune-driven pathophysiology of disease in herpes simplex virus-1 (HSV-1) encephalitis, current treatment paradigms do not officially recommend adjunctive immunomodulatory therapy in addition to acyclovir. This may in part explain the poor long-term outcomes in patients with severe HSV encephalitis. This report is of a 21-year-old man presenting with a 4-day history of nausea, headache, and fever and a diagnosis of HSV-1 encephalitis. CASE REPORT We describe the case of a young male with clinically and radiographically severe HSV-1 encephalitis diagnosed by PCR of cerebrospinal fluid (CSF), who demonstrated immediate improvement upon treatment with intravenous immunoglobulin (IVIG, 0.5 g/kg daily ×3 days) in addition to acyclovir and dexamethasone therapy. Acyclovir therapy was extended beyond 21 days due to persistently positive HSV-1 CSF PCR. He developed N-methyl-D-aspartate (NMDA) receptor antibodies at 6 weeks, but his long-term outcome far exceeded expectations. While some of his neurological deficits appear to be permanent, he is living a normal life. CONCLUSIONS Overwhelming evidence demonstrates that brain injury due to HSV encephalitis is driven by immune reactions stimulated by HSV rather than HSV itself. Nevertheless, use of immunomodulatory therapy such as glucocorticoids and IVIG are left to the discretion of individual clinicians rather than being recommended in treatment guidelines, which instead recommend acyclovir therapy. The present case highlights the potential role of immunomodulatory therapy with IVIG in HSV encephalitis and the importance of early diagnosis and treatment.