Fulminant viral hepatitis (FVH) is a devastating condition caused by hepatotropic viruses such as hepatitis A virus (HAV), hepatitis B virus (HBV), and HSV-1/2. We studied 149 FVH patients (73 males and 76 females, aged 1-76) for blood autoantibodies (auto-Abs) neutralizing type I interferons (IFNs; IFN-α2, -β, -ω). Six of 16 (37.5%) HSV-triggered FVH patients carried such auto-Abs on admission, including three with a previously known autoimmune disease. These patients contrasted with 133 HAV- (n = 46) or HBV-triggered (n = 87) patients, none of whom had such detectable auto-Abs. Odds ratios for HSV-triggered FVH in individuals with auto-Abs ranged from 35.3 (95% CI: 13.0-96.2; P < 10-7) for those neutralizing only 100 pg/ml IFN-α/ω to 1,895 (CI: 448.5-8,002; P < 10-12) for those neutralizing both IFN-α and IFN-ω at 10 ng/ml. Over one third of HSV-triggered FVH cases in this international cohort were due to preexisting auto-Abs. This finding highlights auto-Abs against type I IFNs as a major determinant of HSV-FVH and paves the way for targeted preventive or therapeutic interventions.