Abstract: Objective: To provide a snapshot of the prevalence of abnormal body mass index (BMI) in a sample of Intensive Care Unit (ICU) patients; to identify if any medical specialty was associated with abnormal BMI and to explore associations between BMI and ICU-related outcomes. Methods: Retrospective observational audit of 735 adult patients (median age 58 years) admitted to the Sir Charles Gairdner Hospital 23 bed tertiary ICU between November 2012 and June 2014. Primary outcome measure was patient BMI: underweight (<18.5 kg/m2), normal weight (18.5–24.99 kg/m2), overweight (25–29.99 kg/m2), obese (30–39.99 kg/m2) or extreme obese (40 kg/m2 or above). Other measures included gender, acute physiology and chronic health evaluation II score, admission specialty, length of Mechanical Ventilation (MV), Length of Stay (LOS) and mortality. Results: Compared to the general population there was a higher proportion of obese patients within the cohort with the majority of patients overweight (33.9%) or obese (36.5%) and median BMI of 27.9 (IQR 7.9). There were no significant differences between specialties for BMI (P=0.103) and abnormal BMI was not found to impact negatively on mortality (ICU, P=0.373; hospital, P=0.330). Normal BMI patients had shorter length of MV than other BMI categories and the impact of BMI on ICU LOS was dependent on length of MV. Overweight patients ventilated for five days or more had a shorter LOS, and extremely obese non-ventilated patients had a longer LOS, compared to normal weight patients. Conclusion: Although the obesity–disease relationship is increasingly complex and data presented reflects categorical BMI for patients admitted to a single ICU site it may be important to consider the cost implications of caring for this cohort especially with regard to MV and LOS.