The cause of pediatric acute liver failure (PALF) is unknown in up to 40% of cases. Evidence suggests aberrant immune system activation may play a role. We hypothesized indeterminate PALF cases would exhibit a unique pattern of hepatic inflammation. This was a retrospective and prospective study of PALF cases due to indeterminate (iPALF), autoimmune hepatitis (AIH), or known diagnosis (dPALF) etiology. Liver tissue sections were stained with immunohistochemical (IHC) markers for cytotoxic T-cells (CD8), perforin, and tissue resident memory T-cells (CD103), and scored as minimal, moderate, or dense. Lymphocytes were isolated from liver tissue for T-cell receptor beta (TCRbeta) sequencing and flow cytometry studies. Thirty-three iPALF, 9 AIH, and 14 dPALF cases were included. Dense hepatic infiltrates of CD8+ T-cells were found in 27 (82%) iPALF compared to 1 (7%) dPALF case (p<0.0001). Perforin staining was dense or moderate in 19 (73%) of 26 iPALF cases compared to minimal in all 7 dPALF cases (p=0.004). 16 (62%) of 26 iPALF cases had dense CD103 staining compared to none of the 6 dPALF cases (p=0.001). TCRbeta sequencing of iPALF cases demonstrated increased clonality compared to dPALF and control cases. Flow cytometry and IHC revealed iPALF intrahepatic leukocytes were predominantly tissue resident memory CD8+ T cells. CONCLUSIONS: Indeterminate PALF is characterized by a dense CD8+ T-cell hepatic infiltrate consistent with expansion of a tissue resident memory T-cell phenotype. CD8+ T-cells are a new biomarker of immune dysregulation in iPALF and may be used to better identify and define this group. This article is protected by copyright. All rights reserved.