The primary goal of this work was to develop and evaluate a new method for simultaneous three-dimensional motion estimation and image reconstruction for gated cardiac emission computed tomography (ECT). The method employs a two-step iterative procedure for obtaining the motion and reconstructed image estimates. The method was evaluated using both simulated and physical phantoms designed to mimic myocardial perfusion imaging in ECT. In both the simulated and physical phantom studies, the images reconstructed by the simultaneous method showed improved noise characteristics relative to a standard iterative algorithm. The percent error of the motion estimate ranged from 16% to 59% for the simulated phantom study, and 45% to 65% for the physical phantom study, depending on the position within the myocardium.