Strict glycemic control improves clinical outcomes in critically ill patients. However, practical tools for frequent monitoring of blood glucose (BG) levels in the intensive care unit (ICU) are limited. The Continuous Glucose Monitoring System (CGMS, Medtronic MiniMed, Northridge, CA) is currently approved for detecting glycemic excursions in outpatients with diabetes mellitus. The use of this device has never been carefully examined in the inpatient setting. This preliminary study was designed to investigate the accuracy of the CGMS in critically ill patients admitted to a medical ICU (MICU). Subjects at risk for hyperglycemia were recruited from among all patients admitted to our MICU. CGMS sensors were implanted for up to 72 h. Study subjects wore between one and five consecutive sensors. Four or more standard capillary BG readings were recorded per 24 h. All paired meter-sensor (M-S) readings were used both for CGMS calibration and for data analysis. Twenty-two MICU patients wore 41 CGMS sensors, yielding 546 M-S BG pairs. Overall, the Pearson correlation coefficient ( r ) was 0.88, with a mean M-S difference of 3.3 +/- 26.7 mg/dL (0.6 +/- 17.4%) and a mean absolute M-S difference of 19.7 +/- 18.3 mg/dL (12.8 +/- 11.9%). Clarke Error Grid analysis categorized 98.7% of the M-S pairs within "clinically acceptable" zones A and B. The CGMS is promising for potential use in critically ill patients. If validated in larger studies, the device could serve as a useful research tool for investigating the role of hyperglycemia (and strict glycemic control) in ICU patients. If further developed as a "real-time" glucose sensor, CGMS technology could ultimately prove clinically useful in the ICU, by decreasing nursing workload and/or by providing alarm signals for impending glycemic excursions.