Obesity has been suggested to be a risk factor for increase morbidity and mortality after trauma and surgery. Trauma laparotomy provides an opportunity to assess the effect of body mass index (BMI) on patients subjected to both trauma and surgery. We hypothesized that obesity would have a deleterious effect on outcomes. A retrospective review was conducted of all patients 18 years of age or older undergoing laparotomy for trauma between July 2001 and June 2011. Patients were stratified according to BMI into the following four groups: underweight (16 to 22 kg/m(2)), normal (23 to 27 kg/m(2)), overweight (28 to 34 kg/m(2)), and obese (35 kg/m(2) or higher). Data on the patient's hospital course included length of stay, mortality, respiratory failure, infectious complications, wound dehiscence, and organ failure. A total of 1,297 patients underwent laparotomy. Seven per cent of the study group was obese and 24 per cent was underweight. There was no difference among mean Injury Severity Score, percent of patients arriving in shock, and mean number of units of packed red blood cells administered during their hospital stay. Obese patients had longer intensive care unit and hospital lengths of stay. There were no differences in ventilator days or mortality. Using univariate statistics, obese patients had increased rates of respiratory and renal failure, bacteremia with and without septic shock, and abdominal wound dehiscence. Subjecting the data to logistic regression analysis, BMI was no longer an independent predictor of any complication. Although obese trauma patients do have increased infectious morbidity, wound dehiscence, and a prolonged length of stay, increased BMI is not an independent predictor of increased morbidity or mortality after trauma laparotomy.