OBJECTIVE To show that body composition of intensive care unit patients can be analysed with existing computed tomography (CT) images. We planned to describe changes in visceral fat area (VFA), subcutaneous fat area (SFA) and muscle area (MA) on analysis of specific CT images during acute pancreatitis requiring an ICU admission. DESIGN, SETTING AND PARTICIPANTS Retrospective analysis of body composition using existing CT images, in an ICU of a tertiary university-affiliated hospital 2005-2010, examining 21 patients with acute pancreatitis and CT imaging on two separate occasions within their hospital admission. MAIN OUTCOME MEASURES VFA, SFA, VFA:SFA ratio and MA. Medical records were hand searched to identify ICU and hospital mortalities and other clinical outcomes. RESULTS Three women and 18 men had 84 CT scans analysed, from the level of the right renal hilum and L3 vertebra. The median patient age was 52 years. The median time between CT scans was 9.4 days and the mean Acute Physiology and Chronic Health Evaluation II score was 20.2. ICU mortality was 9%. Analysis showed a decrease in VFA from a median of 229.2 cm2 to 202.1 cm2 (P < 0.01) and a decrease in VFA:SFA ratio from a median of 1.20 to 1.05 (P < 0.01) during the acute illness. MA did not change significantly. CONCLUSIONS The body composition of ICU patients can be analysed through existing CT images. Pancreatitis requiring ICU admission causes a 12% decrease in VFA.