Hypothesis: It is possible to quantify an amount of thoracic hemorrhage, after blunt and penetrating injury, at which delay of thoracotomy is associated with increased mortality. Design: A retrospective case series. Setting: Five urban trauma centers. Study Selection: Patients undergoing urgent thoracotomy (within 48 hours of injury) for hemorrhage (excluding emergency department thoracotomy). Data Extraction: Respective registries identified patients who underwent urgent thoracotomy. Injury characteristics, initial and subsequent chest tube outputs, time before thoracotomy, and outcomes were evaluated. Main Outcome Measure: Death. Results: One hundred fifty-seven patients (36 with blunt and 121 with penetrating injuries) underwent urgent thoracotomy for hemorrhage between January 1, 1995, and December 31, 1998. Mortality correlated with mean (± SD) Injury Severity Score (38 ± 19 vs 22 ± 12.6 for survivors; P<.01) and mechanism (24 [67%] for blunt vs 21 [17%] for penetrating injuries; P<.01). Mortality increased as total chest blood loss increased, with the risk for death at blood loss of 1500 mL being 3 times greater than at 500 mL. Blunt-injured patients waited a significantly longer time to thoracotomy than penetrating-injured patients (4.4 ± 9.0 h vs 1.6 ± 3.0 h; P = .02) and also had a greater total chest tube output before thoracotomy (2220 ± 1235 mL vs 1438 ± 747 mL; P = .001). Conclusions: The risk for death increases linearly with total chest hemorrhage after thoracic injury. Thoracotomy is indicated when total chest tube output exceeds 1500 mL within 24 hours, regardless of injury mechanism.