To compare and correlate left ventricular (LV) myocardial volumes obtained using arterial and delayed phases of multidetector row computed tomography (CT) and evaluate their intra- and interobserver variation. Two observers evaluated the arterial- and delayed-phase serial short-axis images of 45 healthy volunteers. Intra- and interobserver variations in LV myocardial volumes were correlated with four factors—myocardial volume, contrast-volume-to-body-weight ratio, and contrast-to-noise ratios in the arterial and delayed phases. Variations in the apex, mid-ventricle, and base were compared. Intra- and interobserver analyzes revealed no statistical difference and good correlation. Intra- and interobserver variations were within 5 and 10%, respectively, and were independent of the four factors. Variations were the highest at the apex. LV myocardial volumes measured using arterial- and delayed-phase cardiac CT exhibit no significant difference and good correlation. Intra- and interobserver variations are both clinically acceptable, and the apex contributes most to these variations.