OBJECTIVE: To determine to what extent inaccuracies in measuring the end diastolic diameter of the left ventricle, the interventricular septum, and the posterior wall, by M mode echocardiography influence the left ventricular mass calculated by the Devereux's formula. DESIGN: Mathematical model. RESULTS: Relatively small measurement inaccuracies such as 5%, which are known to be inherent in the echocardiographic method, will result in "changes" of left ventricular mass in the range of 8% to 15%. This is equivalent to expected changes in left ventricular mass seen during treatment. CONCLUSIONS: The use of Devereux's formula to calculate left ventricular mass is limited by measurement inaccuracies in individual patients.