The aim of the study was to test whether the mean of the highest and lowest glucose values on day 1 (Glu 1 ) is a useful surrogate marker of mean blood glucose during the totality of intensive care unit (ICU) stay (Glu tot ).Glu tot values were extracted from electronically stored biochemical databases (point-of-care laboratory) and Glu 1 values from electronically stored prospectively collected patient databases in ICUs of 4 hospitals from January 2000 to October 2004. Statistical assessment of relationship between Glu 1 and Glu tot was done.There were 197227 blood glucose measurements for 8039 patients. The average of all blood glucose measurements was 8.22 ± 2.75 mmol/L. The difference between the average of all glucose values (N = 197227) and average of Glu 1 (n = 8039) was 0.17 mmol/L. This difference in each hospital was also small (0.26, −0.13, 0.12, and 0.37 mmol/L, respectively).Glu 1 was a good predictor of Glu tot across all study hospitals. This observation makes it possible to use Glu 1 as a surrogate of glucose control during ICU stay and opens the door to understanding ICU glucose control across the whole of Australia and New Zealand.