We have previously developed and validated a method to achieve quantitative SPECT data based on CT-derived corrections, for the radionuclide (99m)Tc in phantoms and in man for lung scanning. This clinical study was performed to investigate the accuracy of this method when applied to cardiac blood pool imaging. The study involves tagging the radionuclide (99m)Tc to erythrocytes in a sample of the subject's blood before it is re-injected. After a short time, the radiolabelled cells achieve an equilibrium concentration in the blood pool, such that SPECT imaging allows the radioactivity concentration of blood present in the left ventricle to be measured. Absolute concentration of radioactivity inside the left ventricle of the heart was compared to true concentrations measured directly from a peripheral venous blood sample taken from the subject at the time of scanning. In 12 subjects, the average difference between the measured and true concentrations was found to be within approximately 1% of the true value with a range of (-6 to +5)%. This study demonstrates the accuracy of CT-based quantitative SPECT in clinical cardiac blood pool imaging, and we anticipate that similar accuracy could be achieved in the myocardium.