N-acetylcysteine (NAC) was found to improve transplantation-free survival in only those adults with nonacetaminophen (non-APAP) acute liver failure (ALF) and grade 1-2 hepatic encephalopathy (HE). Because non-APAP ALF differs significantly between children and adults, the Pediatric Acute Liver Failure (PALF) Study Group evaluated NAC in non-APAP PALF. Children from birth through age 17 years with non-APAP ALF enrolled in the PALF registry were eligible to enter an adaptively allocated, doubly masked, placebo-controlled trial using a continuous intravenous infusion of NAC (150 mg/kg/day in 5% dextrose in water [D5W]) or placebo (D5W) for up to 7 days. The primary outcome was 1-year survival. Secondary outcomes included liver transplantation-free survival, liver transplantation (LTx), length of intensive care unit (ICU) and hospital stays, organ system failure, and maximum HE score. A total of 184 participants were enrolled in the trial with 92 in each arm. The 1-year survival did not differ significantly (P = 0.19) between the NAC (73%) and placebo (82%) treatment groups. The 1-year LTx-free survival was significantly lower (P = 0.03) in those who received NAC (35%) than those who received placebo (53%), particularly, but not significantly so, among those less than 2 years old with HE grade 0-1 (NAC 25%; placebo 60%; P = 0.0493). There were no significant differences between treatment arms for hospital or ICU length of stay, organ systems failing, or highest recorded grade of HE. CONCLUSION: NAC did not improve 1-year survival in non-APAP PALF. One-year LTx-free survival was significantly lower with NAC, particularly among those <2 years old. These results do not support broad use of NAC in non-APAP PALF and emphasizes the importance of conducting controlled pediatric drug trials, regardless of results in adults.