[en] Three-dimensional data processing was developed for single photon computed tomographic imaging of the multigated blood pool with Tc-99m labelled red blood cells. A gamma camera was rotated from the left posterior oblique to the right anterior oblique projections at intervals of 180 deg/32. Data were collected at each part for one to two minutes, and the number of gates per beat was 12 to 20, depending on the patient. Three-dimensional transaxial images of the heart were reconstructed by the filtered back projection method without attenuation correction. A Shepp and Logan filter was used for the convolution filter. Short-axial images were reconstructed from the transaxial images. Using the equicount level method, the contours of both ventricles at each short-axial cross-section image were determined automatically with 50 % levels of the maximum counts. Each contour line was smoothed to the fifth order of Fourier function after conversion from (x-y) coordinates to (r-θ) polar coordinates with respect to the center of the area. Each contour line, converted again into (x-y) coordinates, was drawn successively on a color CRT at constant intervals and inclinations in order to display the ventricles three-dimensionally. It is possible to display simultaneously on a CRT the right and left ventricles or two types of cardiac disease. It is also possible to depict superimposed images of end-diastolic and end-systolic ventricles. The length of the long axis at end-systole in the normal case became less than half the length of the one at end-diastole. In the case of inferior infarction, the length of the left ventricular long axis did not change according to contraction, and akinesis at the inferior wall was clearly imaged. In the case of hypertrophic cardiomyopathy, the thickness of the interventricular septum was markedly increased compared to that of the normal. (J.P.N.)