Acetaminophen (APAP)-induced Acute Liver Failure (ALF) is associated with significant mortality. Traditional prognostic scores lack sensitivity. Serum Liver-type Fatty Acid Binding Protein (FABP1) early (day 1) or late (day 3-5) levels are associated with 21-day mortality in the absence of liver transplant. Serum samples from 198 APAP-ALF patients (nested case control study with 99 survivors, 99 non-survivors) were analyzed by ELISA methods and assessed with clinical data from the US Acute Liver Failure Study Group (ALFSG) Registry (1998-2014). APAP-ALF survivors had significantly lower serum FABP1 levels early (238.6 vs. 690.8 ng/ml, p <0.0001) and late (148.4 vs. 612.3 ng/ml, p <0.0001) compared with non-survivors. FABP1 > 350 ng/ml was associated with significantly higher risk of death at early (p=0.0004) and late (p<0.0001) time points. Increased serum FABP1 early (log FABP1 odds ratio (OR) 1.31, p=0.027) and late (log FABP1 OR 1.50, p =0.005) were associated with significantly increased 21-day mortality after adjusting for significant covariates (MELD, vasopressor use). Areas under the receiver-operating curve (AUROC) for early and late multivariable models were 0.778 and 0.907 respectively. AUROC of the King's College Criteria (KCC) (Early: 0.552 alone, 0.711 with FABP1; Late: 0.604 alone, 0.797 with FABP1) and ALFSG prognostic index (Early: 0.686 alone, 0.766 with FABP1; Late: 0.711 alone, 0.815 with FABP1) significantly improved with the addition of FABP1 (p <0.002 for all). In patients with APAP-ALF, FABP1 may have good potential to discriminate survivors from non-survivors and may improve models currently used in clinical practice. Validation of FABP1 as a clinical prediction tool in APAP-ALF warrants further investigation. This article is protected by copyright. All rights reserved.