Background: Obesity is a worldwide healthcare concern, but its impact on critical care (intensive care unit; ICU) outcomes is not well understood. The general hypothesis is that obesity worsens ICU outcomes, but published reports fail to demonstrate this effect consistently. We hypothesized that increasing BMI would be an independent predictor of higher mortality rates in the surgical/trauma ICU. Methods: Data on patients with infections, defined by U.S. Centers for Disease Control and Prevention criteria, were collected prospectively from a single university surgical/trauma ICU. From 1996 to 2003, 807 such patients had measurable BMIs on admission to the ICU and were divided into underweight (<18.5 kg/m2), normal weight (18.5-24.9 kg/m 2), overweight (25.0-29.9 kg/m2), obese (30.0-39.9 kg/m2), and morbidly obese (≥40.0 kg/m2). The primary outcome was in-hospital death. Bivariate and multivariate analyses were performed. Results: In-hospital death was associated with increasing age, increasing average Acute Physiology and Chronic Health Evaluation (APACHE) II score, history of diabetes (p = 0.001), cardiac disease (p = 0.001), hypertension (p = 0.044), history of cerebrovascular disease (p = 0.021), renal insufficiency (p = 0.007), need for hemodialysis (p < 0.001), history of pulmonary disease (p = 0.012), requirement for mechanical ventilation while in the ICU (p = 0.107), history of malignant disease (p < 0.001), and history of liver disease (p < 0.001). The multivariate analysis selected age (odds ratio [OR] 1.03 per integer; confidence interval [CI] 1.0, 1.05), APACHE II score (OR 1.17 per integer; CI 1.12, 1.74), diabetes (OR 2.20; CI 1.32, 3.65), mechanical ventilation (OR 1.88; CI 1.21, 2.94), malignancy (OR 2.54; CI 1.43, 4.47), and liver disease (OR 5.01; CI 2.69, 9.32) as significant risk factors. When controlling for these variables, none of the BMI groups had an independent association with death compared with the normal weight group. Conclusion: Contrary to the hypothesis, the data suggest no discernable independent association of increasing BMI with heightened mortality rate in the surgical/trauma ICU patient with infection.