BACKGROUND The feasibility of determining left ventricular (LV) ejection fraction (EF) from 99mTc-labeled sestamibi gated tomography (GSPECT) is well established. To improve precision of measurement, rules used by observers in processing tomograms were encoded for automation. RESULTS LV centers were estimated from activity centroids of time-difference images exceeding 50% of maximum counts. End diastole and end systole were defined by time-varying maximum count extremes. Endocardial borders were generated by fitting maximum locations with fifth-order two-dimensional harmonics, searching inward to predetermined thresholds, and reconciling endocardial with valve plane points. Regression analysis of GSPECT EF yielded r = 0.87 versus equilibrium gated blood pool in 75 patients and r = 0.87 versus gated first pass in 65 patients. GSPECT EF interobserver variability was r = 0.92 and intraobserver automatic versus manual linear correlation was r = 0.94. A subgroup of 25 studies was analyzed by six independent observers, for whom EF agreement with the core laboratory ranged from r = 0.93 to r = 0.96. Experienced observers judged it necessary to alter end-diastolic or end-systolic frames in 7% of patients, endocardial borders in 14%, and LV centers in 28%. CONCLUSIONS Results of automated GSPECT LV EF correlated well with those of manual GSPECT and gated first-pass and equilibrium blood pool values and were highly reproducible.