Primary and secondary abdominal compartment syndrome (ACS) are well-recognized entities after trauma. The current study describes a “hyperacute” form of secondary ACS (HACS) that develops intraoperatively while repair of extra-abdominal injuries is being carried out simultaneous with massive resuscitation for shock caused by those injuries. The charts of patients requiring abdominal decompression (AD) for HACS at time of extra-abdominal surgery at our level I trauma center were reviewed. The following data was gathered: age, Injury Severity Score (ISS), mechanism, resuscitation details, time to AD, time to abdominal closure, and outcome. All continuous data are presented as mean ± standard error of mean. Hemodynamic and ventilatory data pre- and post-AD was compared using paired t test with significance set at P < 0.05. Five (0.13%) of 3,750 trauma admissions developed HACS during the 15-month study period ending February 2004. Mean age was 32 ± 7 years, and mean ISS was 19 ± 2. Four of five patients arrived in hemorrhagic shock (blunt subclavian artery injury, 1; chest gunshot, 1; gunshot to brachial artery, 1; stab transection of femoral vessels, 1) and were immediately operated upon. One of five patients (70% burn) developed HACS during burn wound excision on day 2. HACS developed after massive crystalloid (15 ± 1.7 L) and blood (11 ± 0.4 units) resuscitation during prolonged surgery (4.8 ± 0.8 hours). Pre- versus post-AD comparisons revealed significant ( P < 0.05) improvements in mean arterial pressure (55 ± 6 vs 88 ± 3 mm Hg), peak airway pressure (44 ± 5 vs 31 ± 2 mm Hg), tidal volume (432 ± 96 vs 758 ± 93 mL), arterial pH (7.16 ± 0.0 vs 7.26 ± 0.04), and PaCO2 (52 ± 6 vs 45 ± 6 mm Hg). There was no mortality among the group, and all patients underwent abdominal closure by fascial reapproximation in 2–5 days. Two (40%) of the five patients required extremity fasciotomy for compartment syndrome. HACS is a rare complication of massive resuscitation for extra-abdominal injuries. It should be considered in such patients in the face of unexplained hemodynamic and/or ventilatory decompensation. Prompt AD is life saving. Early abdominal closure is usually possible. Vigilance for compartment syndromes elsewhere in the body is warranted in any patient with HACS.)