The total intrinsic variability of echocardiographically determined left ventricular muscle mass was evaluated in this study. Cross-sectional left ventricular echocardiograms were made in 13 normal volunteers (8 men, 5 women), ages 31 to 41 years (mean 36) with an interval period of about eleven months. We used the parasternal long axis view, the short axis view at the papillary muscle level and the four-chamber view from the apex. M-mode recordings were made from the parasternal long-axis view. Left ventricular muscle mass from M-mode recordings was calculated using a simple cube formula and from cross-sectional echo cardiography with an area-length formula. The reproducibility of the left ventricular muscle mass for the methods was respectively 10 +/- 6% and 5 +/- 3% (mean +/- SD). From both methods cross-sectional echo is the method of choice for determining left ventricular muscle mass in man in sequential studies.