The objective of this study was to identify left atrial (LA) abnormality on the electrocardiogram and other related variables as predictors of left ventricular (LV) hypertrophy in the presence of left bundle branch block (LBBB). In the presence of complete LBBB, the diagnosis of electrocardiographic abnormalities is problematic and that of LV hypertrophy remains difficult. The usual electrocardiographic criteria applied for the diagnosis of LV hypertrophy may not be reliable in the presence of LBBB. Therefore, noninvasive criteria will help physicians diagnose LV hypertrophy with electrocardiography. LA abnormality on the electrocardiogram was assessed by 2 independent observers as predictor of LV hypertrophy in the presence of LBBB in 120 patients, and data were compared with those of 100 patients without LA abnormality. LV mass was calculated from echocardiographic data. Besides LA abnormality, the other variables studied for prediction of LV hypertrophy were gender, age, body surface area, body mass index, frontal axis, and QrS duration. Of the 6 criteria analyzed, the P terminal force was found to be the most common and consistent criterion to detect LA abnormality. LV hypertrophy was confirmed by echocardiographic determination of LV mass in both groups. Observers reliably differentiated between the hypertrophied and normal-sized left ventricle in the presence of LBBB by correlating LA abnormality with LV mass determined by echocardiography. Observer 1 detected LA abnormality in 89% and observer 2 in 84% of patients. False-positive results were present in 11% and 16%. The observer's recognition of LA abnormality in the present study was 91%. The 2 observers showed a sensitivity of 81% and 79% and a specificity of 91% and 88%, respectively, when diagnosis of LV hypertrophy was determined. LV mass increased significantly and was diagnostic of LV hypertrophy in 92% of patients with LA abnormality. In the remaining 11 patients (8%), the LA abnormality was of marginal abnormal magnitude. Each 0.01-mV/s increase in LA abnormality gave an increase of 30 g of LV mass. LV mass was increased in 86% of patients when corrected by body surface area. LV hypertrophy in the presence of LBBB on electrocardiography was found in only 13 patients (10%) when the 6 frequently used conventional criteria for diagnosis of LV hypertrophy by electrocardiography were used. Regression analysis revealed LA abnormality to be a strong independent predictor of increased LV mass. Multivariate analysis also revealed age, body mass index, body surface area, frontal axis, and QrS duration to be significant predictors of LV mass. This noninvasive study correlates LA abnormality by electrocardiogram and LV hypertrophy with echocardiography to conclude that LA abnormality was significantly diagnostic of LV hypertrophy in the presence of LBBB. Age, body mass index, body surface area, frontal axis, and QrS duration were also significant predictors of LV mass.