To test the hypothesis that single-photon emission computed tomography (SPECT) could actually determine left ventricular LV mass in humans, SPECT measurements of LV mass were compared with LV mass determined by cineangiography in 12 patients with normal coronary arteries and LV function. Repeat SPECT determinations of LV mass were carried out in 5 patients. Projection images of the left ventricle were acquired after intravenous injection of thallium-201 (TI-201) using a rotating gamma camera. Transverse sections were reconstructed by filtered backprojection. The boundary of LV uptake of TI-201 in each transverse section was defined using a 3-dimensional threshold detector. Scintigraphic LV mass (total number of voxels demonstrating LV TI-201 uptake X voxel volume X specific gravity of myocardium) was compared with angiographic LV mass. There was good correlation between LV mass determined by SPECT and that determined by cineangiography. Mean angiographic LV mass was 208 +/- 45 g (+/- standard deviation). Mean SPECT LV mass was 204 +/- 42 g. Linear regression analysis revealed the following relation: SPECT LV mass = 0.76 X angiographic LV mass + 46.1 (r = 0.82, root-mean-square deviation from regression = 24.7). The SPECT values of LV mass varied an average of 10.4 +/- 4.6% (+/- standard deviation) in the 5 patients in whom 2 determinations were made. Thus, SPECT of TI-201 can accurately measure LV mass in humans.