Abstract-The study of left ventricular (LV) hypertrophy is hindered by problems with LV mass measurement by echocardiography. Both the M-mode and 2D area-length formulas for calculating LV mass assume a fixed geometric shape, which may be a source of error. We examined this hypothesis by using cardiovascular magnetic resonance images to eliminate the confounding effects of acoustic access and image quality. LV mass was measured directly in 212 healthy subjects by means of a standard 3D cardiovascular magnetic resonance technique. LV mass was also calculated by using the cube-function and area-length formulas with measurements from the magnetic resonance images. A comparison of serial measurements was made by examining the changes in LV mass by all 3 techniques in those completing an exercise program (nϭ140). The cube-function technique showed a consistent underestimation of LV mass of 14.3 g, and there were wide 95% limits of agreement (Ϯ57.6 g and Ϯ46.3 g for cube-function and area-length techniques, respectively) when compared with 3D measurement. There were similarly wide limits of agreement for the change in mass (Ϯ55.2 g and Ϯ44.8 g for cube-function and area-length, respectively). The assumption of geometric shape in the cube-function and area-length formulas resulted in significant variation in LV mass estimates from direct measurement by using a 3D technique. Key Words: myocardium Ⅲ hypertrophy Ⅲ magnetic resonance imaging Ⅲ echocardiography L eft ventricular (LV) hypertrophy is an independent cardiovascular risk factor associated with significant excess and morbidity and mortality rates. [1][2][3] There is now evidence for the effectiveness of antihypertensive agents, particularly ACE inhibitors, in reducing LV mass, 4 -6 and this reduction in LV mass appears to carry a favorable prognosis. 7,8 However, studying the prognostic implications of LV mass reduction is hindered by the poor accuracy and reproducibility of LV mass measurement by M-mode and 2D echocardiography. The high 95% confidence limits for accuracy (Ϯ57 to 190 g for M-mode, 9 -12 Ϯ61 to 80 g for 2D techniques [11][12][13] ) and interstudy reproducibility (Ϯ45 to 78 g for M-mode, 10,14 -17 poorly assessed for 2D) result in the need for large numbers of subjects in research studies.The principal sources of error are considered to be image quality, beam positioning, and the assumption of a uniform geometric shape of the left ventricle, with LV mass calculated from measurements made at 1 or 2 positions. Both M-mode and 2D echocardiography assume a prolate ellipsoid shape, with a ratio of long-to short-axis lengths of 2:1, which provided the best simplified geometric model for LV mass estimation. 18 Formulas were developed for the calculation of LV mass based on the regression equations of the calculated mass to autopsy findings for M-mode 9,10 and 2D echocardiography. 12,19 -21 The cubing involved in the formulas means that small (Ͻ1 mm) differences in measurement can have large effects on the calculated mass. Additionally, the assumption o...