STUDY OBJECTIVE: Obese patients seem to have worse outcomes and more complications during intensive care unit (ICU) stay. This study describes the clinical course, complications and prognostic factors of obese patients admitted to an intensive care unit compared to a control group of nonobese patients. DESIGN: Retrospective observational study. SETTING: A 10-bed adult intensive care unit in a university-affiliated hospital. METHODS: All patients admitted to the intensive care unit over 52 months (April 01/2005 to November 30/2008) were included. Obese patients were defined as those with a body mass index (BMI) ≥ 30 Kg/M2. Demographic and intensive care unit related data were also collected. An clinical and demographical matching group of eutrophic patients selected from the data base as comparator for mortality and morbidity outcomes. The Mann-Whitney test was used for numeric data comparisons and the Chi Square test for categorical data comparisons. RESULTS: Two hundred nineteen patients were included. The obese group (n=73) was compared to the eutrophic group (n= 146). Most of this group BMI ranged between 30 - 35 Kg/M2. Only ten patients had body mass index ≥40 Kg/M2. Significant differences between the obese and eutrophic groups were observed in median APACHE II score (16 versus 12, respectively; p<0.05) and median intensive care unit length of stay (7 versus 5 days respectively; p<0,05). No significant differences were seen regarding risk of death, mortality rate, mechanical ventilation needs, days free of mechanical ventilation and tracheostomy rates. The observed mortality was higher than the APACHE II-predicted for both groups, but the larger differences were seen for morbid obese patients (BMI ≥40 Kg/M2). CONCLUSIONS: Obesity did not increase the mortality rate, but improved intensive care unit length of stay. The current prognostic scoring systems do not include BMI, possibly underestimating the risk of death, and other quality of care indexes in obese patients. New studies could be useful to clarify how body mass index impacts the mortality rate.