The presence of left bundle branch block (LBBB) on 12-lead ECG may obscure the diagnosis of other ECG abnormalities including left ventricular hypertrophy (LVH). We retrospectively reviewed ECGs of patients with LBBB and LVH as determined by echocardiography to evaluate several ECG parameters as predictors of LVH. ECG evaluation included precordial voltage as measured by the sum of the S wave in leads V1 or V2 plus R wave in lead V6, QRS duration, mean frontal plane QRS axis, R wave amplitude in lead aVL, intrinsicoid deflection, and the presence or absence of criteria for left atrial enlargement. In the presence of LBBB and LVH, precordial voltage was significantly greater (p less than 0.001), QRS duration more prolonged (p less than 0.001), and left atrial enlargement more frequently present (p less than 0.001) than when LVH was not present. There was no difference in limb lead voltage, intrinsicoid deflection, or mean frontal plane QRS axis. Furthermore, the criterion of SV2 + RV6 greater than 4.5 mV demonstrated a sensitivity of 86% and a specificity of 100%. We conclude that a voltage criterion of SV2 + RV6 greater than 4.5 mV is diagnostic of LVH in the presence of LBBB; furthermore, QRS duration of greater than 160 msec plus left atrial enlargement strongly supports the diagnosis of LVH.