Objectives:To develop and evaluate a method for measuring blood volume using the dilution of a fluorescent-labeled hydroxyethyl starch.Design:Laboratory and clinical investigation.Setting:Biochemistry laboratory at the University of Cardiff. Hematology clinic, surgical ward and intensive care unit of the University Hospital of Wales.Patients:Seventeen patients with suspected polycythemia. Eight patients who had undergone major surgery and/or were receiving intensive postoperative care.Interventions:All surgical and postoperative care was provided by clinicians not involved in the study. Patients with suspected polycythemia were referred for blood volume measurement using labeled albumin and red blood cells.Measurement and Main Results:A proprietary brand of hydroxyethyl starch (Elohaes) was labeled with fluorescein isothiocyanate. Dilution of this compoundin vivowas used for measuring blood volume, and the results were compared with those obtained using radiolabeled albumin and the considered criterion, radiolabeled red cells. The elimination of the labeled starch follows the same progress as that of the parent compound, indicating that the fluorescent tag is stable in vivo. The volume of distribution of the labeled starch is 2.5 mL/kg lower than that for labeled albumin (p= .05). Blood volume, measured from the dilution of fluorescent starch, is lower (4.9 mL/kg) than that measured with albumin (p= .048) but higher (6.61 mL/kg) than that measured with red blood cells (p= .0007). This latter difference may be even smaller at marginally higher doses of the fluorescent starch.Conclusion:These data support the view that hydroxyethyl starch provides a valid alternative to red cell labels as a means of calculating blood volume in patients. Labeling the starch with a fluorescent marker makes the assay procedure more sensitive and infinitely easier. The dose required is not high enough to affect the hemodynamic status of the patient.