OBJECTIVES: To assess mechanism of injury as a clinical course predictor in patients with complete anterior and posterior pelvic ring disruptions [innominosacral dissociation (ISD)]. DESIGN: Retrospective review of radiographs and medical data. SETTING: R Adams Cowley Shock Trauma Center, Baltimore, Maryland, statewide trauma center. PATIENTS: Forty-three patients with ISD were admitted to our institution between August 1986 and October 1991. Five patients were excluded because of incomplete medical records or refusal of blood transfusion. INTERVENTION: Injuries were grouped according to the Young classification: 18 anteroposterior compression (APC), 14 vertical shear (VS), and 6 other injuries. MAIN OUTCOME MEASUREMENTS: The mean blood replacement requirements, incidence of multiple organ system failure, mortality rate, and length of hospital stay for each injury classification were compared. RESULTS: The mean ISS was 34, and the mean 24-hour packed red blood cell transfusion requirement was 12.6 units. Thirteen patients (34.4%) developed multisystem organ failure. Eight patients (21%) died. Patients in the APC group were more likely to require > or = 10 units of blood (15/18, p = 0.001, and those in the VS group were more likely to receive <10 units (11/14, p = 0.0014). Multisystem organ failure occurred more frequently (11/18 versus 2/14; p < 0.005), mortality was significantly higher (39 versus 0%, respectively; p = 0.01), and mean hospital stay for survivors was longer (48 versus 27 days; p < 0.025) in the APC than in the VS group, respectively. CONCLUSIONS: These findings suggest that mechanism of injury is an important determinant of clinical behavior in patients with IDS, and that ISD secondary to the APS mechanism is associated with substantially greater resuscitation requirements, morbidity, and mortality than ISD secondary to the VS mechanism.