['We noted a bimodal relationship between mortality and shock index (SI), the ratio of heart rate to systolic blood pressure.', 'To determine if extremes of SI can predict mortality in trauma patients.', 'Retrospective evaluation of adult trauma patients at a tertiary care center from 2000 to 2012 in the United States.', 'We examined the SI in trauma patients and determined the adjusted mortality for patients with and without head injuries.', 'Descriptive statistics and multivariable logistic regression.', 'SI values demonstrated a U-shaped relationship with mortality. Compared with patients with a SI between 0.5 and 0.7, patients with a SI of <0.3 had an odds ratio for death of 2.2 (95% confidence interval [CI] 21.2-4.1) after adjustment for age, Glasgow Coma score, and injury severity score while patients with SI >1.3 had an odds ratio of death of 3.1. (95% CI 1.6-5.9). Elevated SI is associated with increased mortality in patients with isolated torso injuries, and is associated with death at both low and high values in patients with head injury.'] Does extremes of shock index predict death in trauma patients?