BACKGROUND: More than half of children with pediatric acute liver failure (PALF) experience hepatic encephalopathy (HE), which is related to poor outcomes; however, HE is difficult to diagnose in children. The objective of this study was to evaluate if heart rate variability (HRV), a continuous measure of autonomic nervous system function, was related to the presence and severity of HE as well as clinical outcomes in children with PALF. METHODS: We conducted a retrospective observational cohort study of 38 critically ill children with PALF to examine the association between HRV and HE severity and clinical outcome. HRV was estimated using the integer HRV (HRVi). Categorical variables were compared using the Fisher Exact test and continuous variables were compared using Kruskal-Wallis tests. Associations between grades of HE and minimum and median HRVi were evaluated with Pearson\'s correlation, with p values <0.05 considered significant. RESULTS: A more negative median and minimum HRVi, indicating poorer autonomic nervous system function, was significantly associated with abnormal EEG findings, presence of HE, and poor outcomes (death or listing for transplant). CONCLUSIONS: Heart rate variability may hold promise to predict outcomes in children with PALF, but these findings should be replicated in a larger sample. IMPACT: The findings of our study suggest that heart rate variability is associated with clinical outcomes in children with acute liver failure, a cohort for which prognostics are challenging, especially in young children and infants. Use of heart rate variability in the clinical setting may facilitate earlier detection of children with pediatric acute liver failure (PALF) at high risk for severe hepatic encephalopathy and poor outcomes. Identification of children with PALF at high risk of decompensation may assist clinicians in making decisions about liver transplantation, an important, but resource-limited, treatment of PALF.