This study assessed the capability of ECG-gated MR imaging for quantitating left ventricular mass by means of signal intensity-based and geometric methods for measuring left ventricular mass of normal dogs and dogs with left ventricular hypertrophy. Mass was measured on transverse images encompassing the left ventricle during both diastole and late systole. Partial-volume errors were minimized by measuring the length of the left ventricle on a sagittal image and weighting the mass of end slices accordingly. The range of postmortem left ventricular mass was 61-100 g. The linear relationship between postmortem left ventricular mass and mass measured via MR images correlated closely when MR imaging measurements were done at either end diastole (r = .94) or late systole (r = .94). The standard errors of the estimate were 13.7 and 14.7 g for images gated to end diastole and late systole, respectively. Inter- and intraobserver reproducibility showed excellent agreement (r = .93 and r = .89 for end diastole and r = .99 and r = .93 for late systole, respectively). Thus, left ventricular mass can be quantified accurately and reproducibly over a wide range of masses by using ECG-gated MR imaging.