The ‘renal threshold for glucose’ has never been evaluated in critically ill patients. Therefore, we aimed to investigate the renal glucose threshold in this patient group using high-sensitivity urine glucose assays. In this retrospective analysis of prospectively collected data, we analysed 100 consecutive critically ill patients from a medical intensive care unit (ICU). Arterial blood glucose and spot urine glucose were simultaneously quantified daily during the first week after ICU admission. Three hundred seventy-three pairs of blood/urine glucose were plotted in five pre-defined categories of blood glucose (<80, 80–109, 110–139, 140–179 and ≥180 mg/dL). Urine glucose values of the five categories were compared using the Kruskal–Wallis test to assess the relation with blood glucose. Urine glucose was detected in virtually all of the urine samples. Urine glucose showed a positive nonlinear correlation with blood glucose and was significantly elevated at blood glucose levels of 140–179 and ≥180 mg/dL compared with lower blood glucose ranges. Basal glucosuria is ubiquitous in critically ill patients. A ‘soft’ renal threshold for glucose is present at blood glucose levels in the range of 140–179 mg/dL.