Background/Aims: Obesity is a worldwide concern, but its influence on critical care outcomes is not well understood. We tested the hypothesis that abnormal body mass index (BMI) would be an independent predictor of higher mortality rates in intensive care unit (ICU). Methods: We retrospectively reviewed patients who had admitted to the ICU from January 2007 to December 2007. Admission BMI was analyzed as both a three categorical (underweight, <18.5 kg/m2; normal weight, 18.5 to 24.9 kg/m2; overweight and obese, ≥25 kg/m2) and continuous variables among all patients with an ICU lenth of stay ≥4 days. The primary outcome was ICU mortality. Results: The multivariate analysis on ICU mortality selected Mortality Prediction Model-Admission (MPM at time zero) (hazard ratio [HR], 1.024; p=0.001; 95% confidence interval [CI], 1.010 to 1.037), failed extubation (HR, 5.092; p=0.0001; 95% CI, 2.742 to 9.456) as significant risk factors. When controlling these variables, none of the BMI group and BMI as a continuous variable had an independent association with ICU mortality. Conclusions: BMI did not have a significant influence on ICU mortality. The ICU mortality was influenced more strongly by severity of illness and failed extubation rather than BMI. (Korean J Intern Med 2010;25:162-167)