Corticosteroids reduce insulin sensitivity in healthy individuals by 30- 62-percent. The aim of this research was to use model-based methods to determine whether this reduction is also true in critically ill patients and how it may affect tight glycaemic control. A clinically validated model-based measure of insulin sensitivity was used to quantify changes between two matched cohorts of 40 intensive care unit (ICU) patients from Christchurch hospital. A 9-percent reduction in median insulin sensitivity was seen between the control cohort and patients receiving corticosteroids (per patient dose equivalent to 160mg/d of hydrocortisone). On a per-patient basis 11- 22-percent reductions were observed with higher percentile patients having greater suppression of insulin sensitivity. This research has shown that corticosteroids cause a much lower reduction in insulin sensitivity for critically ill patients compared to healthy controls and may thus have far less impact than suspected on glycaemic control in the ICU setting.