Objective: Our experimental studies have indicated that the initial distribution volume of glucose (IDVG) has potential to evaluate the central extracellular fluid (ECF) volume status in the body. The purpose of the present study was to compare the IDVG and cardiac output (CO) in critically ill patients with or without expansion of the central ECF volume and to test whether a larger IDVG is observed during the presence of increased central ECF volume than during the absence of it. Design and Setting: The study was prospective and was conducted in the intensive care unit, University of Hirosaki Hospital, after approval of the institutional ethical committee. Patients: Fourteen patients with accumulation of the central ECF volume (the accumulation group) and 29 patients without it (the non-accumulation group) were studied. Interventions: Glucose 5 g was infused over 30 s when the patients’ hemodynamic states were relatively stable. Blood samples were taken immediately before infusion and during 7 min post-infusion. Thermodilution CO was also measured before each glucose challenge. A total of 81 comparisons were performed. Measurements and Results: The IDVG was calculated using a one-compartment model. A linear correlation was obtained between the IDVG and CO in the non-accumulation group (r = 0.89, n = 54, p < 0.001). The IDVG-CO ratio was higher in the accumulation group than in the non-accumulation group (p < 0.01). The increased plasma glucose concentrations at 3 min postinfusion were inversely proportional to the IDVG (r = -0.90, n = 81, p < 0.001). Conclusions: Results demonstrated that a larger IDVG was obtained during accumulation of the central ECF volume and support the hypothesis that the IDVG reflects the central ECF status in critically ill patients characterized in our study, even though CO influences the IDVG.