Hypothesis: Grade 4 and grade 5 blunt liver injuries can be safely treated by nonoperative management (NOM). Design: Retrospective case series. Setting: Eleven level I and level II trauma centers in New England. Patients: Three hundred ninety-three adult patients with grade 4 or grade 5 blunt liver injury who were admitted between January 1, 2000, and January 31, 2010. Main Outcome Measure: Failure of NOM (f-NOM), defined as the need for a delayed operation. Results: One hundred thirty-one patients (33.3%) were operated on immediately, typically because of hemodynamic instability. Among 262 patients (66.7%) who were offered a trial of NOM, treatment failed in 23 patients (8.8%) (attributed to the liver in 17, with recurrent liver bleeding in 7 patients and biliary peritonitis in 10 patients). Multivariate analysis identified the following 2 independent predictors of f-NOM: systolic blood pressure on admission of 100 mm Hg or less and the presence of other abdominal organ injury. Failure of NOM was observed in 23% of patients with both independent predictors and in 4% of those with neither of the 2 independent predictors. No patients in the f-NOM group experienced life-threatening events because of f-NOM, and mortality was similar between patients with successful NOM (5.4%) and patients with f-NOM (8.7%) (P=.52). Among patients with successful NOM, liver-specific complications developed in 10.0% and were managed definitively without major sequelae. Conclusions: Nonoperative management was offered safely in two-thirds of grade 4 and grade 5 blunt liver injuries, with a 91.3% success rate. Only 6.5% of patients with NOM required a delayed operation because of liver-specific issues, and none experienced life-threatening complications because of the delay.