Abstract This study tests the electrocardiographic diagnosis of left ventricular (LV) hypertrophy in the presence of left bundle branch block (BBB). The LV mass of 125 patients with left BBB was estimated by echocardiography. M-mode echocardiography was technically adequate in 80% of patients. LV mass was calculated using previously validated M-mode formulas and then indexed to body surface area. The known shifts in the QRS voltage and axis with the onset of left BBB led to the selection of 4 electrocardiographic parameters for the diagnosis of LV hypertrophy: R in aVL 11 or more; QRS axis −40 ° or less (or SII > RII); SV 1 + RV 5 to RV 6 40 or more; SV 2 30 or more and SV 3 25 or more; these parameters were used in cumulative fashion. This cumulative approach was superior to using single conventional criterion such as the SV 1 + RV 5 or RV 6. When LV hypertrophy was defined as an M-mode index of at least 115 g/m 2, the sensitivity was 75% and specificity 90%. Using an M-mode mass of at least 215 g as the standard, the sensitivity was 73% and the specificity 66%. LV hypertrophy can be diagnosed by electrocardiographic criteria in the presence of left BBB at least as reliably as in normal conduction.