The impact of obesity on critical care outcomes has been an issue for debate in the literature. Variable data and conflicting results have been reported. The purpose of our study is to examine the impact of obesity on the outcome of patients admitted to a tertiary closed Intensive Care Unit (ICU) in Saudi Arabia. Data was obtained from a prospectively collected database from September 2001 to May 2004. Patients younger than 18, those with burns, brain death and readmissions were excluded. The study population was stratified into six groups according to their Body Mass Index (BMI). Primary endpoints were ICU and hospital mortality, duration of mechanical ventilation and ICU length of stay. A total of 1835 patients were included in the analysis. Baseline characteristics were similar among the six groups including severity of illness scores, reflected by Acute Physiology and Chronic Health Evaluation II (APACHE II) scores. The ICU mortality was not statistically different among the groups. Hospital mortality was lower in patients with BMI 35-39.9 kg/m2 and BMI >40 kg/m2 compared to those with BMI 18.5-24.9 kg/m2. Multivariate analysis showed that a BMI >40 kg/m2 was an independent predictor of lower hospital mortality (odds ratio 0.51, 95% confidence interval 0.28-0.92, P 0.025) after adjustment for other confounding factors. In conclusion, mortality of obese critically ill patients was not higher than patients with normal weight. In fact, the hospital mortality was lower for patients with BMI >40 kg/m2 compared to the normal BMI group despite similar severity of illness. Obesity might have a protective effect, although further studies are needed to substantiate this finding.