It is generally accepted in clinical electrocardiography that a right bundle branch block (RBBB) does not interfere with the electrocardiographic (ECG) diagnosis of myocardial infarction (MI). The basic assumption is that the initial excitation wavefronts are relatively unchanged in RBBB. This study compared serial changes in Q wave duration in inferior leads II, III, and aVF in 9 patients who developed RBBB within 3 weeks after myocardial revascularization procedure (RBBB group) and in 41 revascularized patients without RBBB in the same observation period (control group). Q wave durations in the electrocardiograms obtained before the patients' procedures were not significantly different between the study and control groups. However, Q wave durations shortened significantly more in the RBBB group than in the control group. The most pronounced Q wave duration shortening took place in lead aVF, -18.2 ms in the RBBB group versus -3.8 ms in the control group (P = .0001). The shortening was less pronounced, although significant, in leads II and III: II, -7.6 +/- -10.9 ms in the RBBB group vs -2.3 +/- -3.5 ms in the control group (P = .01); III, -11.3 +/- -10.5 ms vs -2.6 +/- -6.5 ms (P = .002); aVF, -18.2 +/- -13.5 ms vs -3.8 +/- -5.3 ms (P < .0001). It is concluded that incident RBBB complicating revascularization procedures may cause significant alterations in spatial orientation of the initial excitation wavefronts. This may be a potential source of false-negative ECG diagnosis of inferior MI, particularly in clinical trials where serial ECG analysis is an important part in MI classification.