Because of its simplicity, widespread availability, relatively low cost, and lack of adverse effects, M-mode echocardiography has become the most widely used technique for measurement of human left ventricular mass. Necropsy comparison studies have yielded formulas for anatomically accurate estimation of left ventricular mass in patients with normally shaped ventricles using left ventricular measurements by either Penn or American Society of Echocardiography conventions, but M-mode methods are less accurate in abnormally shaped ventricles. The standard error of M-mode echocardiographic left ventricular mass measurements is approximately 40 g under difficult clinical recording conditions and 30 g or less for research studies of stable subjects. Interstudy variability of mass estimates appeared somewhat lower, resulting in 95% confidence limits for serial change up to 58 g for individual subjects and up to 10 g for study populations of 34 patients or more. The accuracy of M-mode echocardiography for measurement of left ventricular mass is similar to that of contrast angiography but may be exceeded by newer methods with greater cost or radiation exposure, including magnetic resonance imaging, cine-computed tomography, and three-dimensional echocardiographic reconstruction. Identification of left ventricular hypertrophy needs to take into account the influence of sex and body size, the variables that most influence normal ventricular mass, with provisional criteria for recognition of hypertrophy being left ventricular mass index over 134 g/m2 in men and above 110 g/m2 in women.