Background Left ventricular function in patients with right bundle branch block is variable and depends on the population under study. This study assessed the implications of right bundle branch block for the estimation of resting left ventricular function in patients with right bundle branch and suspected coronary artery disease. Methods and Results Seventy-four patients with right bundle branch block, symptoms suggestive of coronary artery disease, and no electrocardiographic Q waves were compared with 649 patients with entirely normal electrocardiograms to assess the implications of right bundle branch block on resting left ventricular function. Resting ejection fraction was determined by radionuclide ventriculography. Patients with right bundle branch block were older (mean 65.0 ± 10.2 years vs 53.8 ± 11.1; P < .001) and had a lower mean ejection fraction (60% ± 11% vs 63% ± 9%; P < .005) compared with patients with normal electrocardiograms. There was a highly significant interaction between right bundle branch block and sex with respect to resting ejection fraction (P < .001). The mean ejection fraction for men with right bundle branch block was 57% ± 10% (17% with abnormal resting ejection fraction) compared with 62% ± 8% (7% with abnormal resting ejection fraction) for normal men. In contrast, the mean ejection fraction for women with right bundle branch block was 68% ± 9% (0% with abnormal resting ejection fraction) compared with 65% ± 9% (5% with abnormal resting ejection fraction) for normal women. Conclusions Male patients with right bundle branch block and symptoms suggestive of coronary artery disease have a lower resting ejection fraction than male patients with normal electrocardiograms. This difference is not seen in female patients. (Am Heart J 1998;136:418-24.)