Exercise-induced myocardial ischemia is difficult to detect with ST-T changes in patients with right bundle branch block (RBBB). We sought to predict exercise-induced myocardial ischemia with QT interval behavior during exercise in patients with RBBB. Twenty-two patients with angiographically proven coronary artery disease and RBBB and 9 healthy volunteers underwent nitrogen-13 ammonia positron emission tomography with bicycle ergometer exercise at a fixed workload of 25 W. Regional myocardial blood flow (RMBF) and electrocardiographic changes were measured both at rest and after 5 minutes of exercise. The QT interval was measured from the onset of the QRS complex to the offset of the T wave in lead V₅. The ΔQT and ΔRMBF, which indicated values after 5 minutes of exercise minus values at rest, were negatively correlated (r = −0.74, p <0.001). Exercise-induced shortening of the QT interval (422 ± 27 to 381 ± 38 ms, p = 0.0020) was observed in 15 patients (group 1) and no change or prolongation (411 ± 45 to 420 ± 37 ms, p = NS) was observed in 7 patients (group 2). Multivessel disease was significantly more frequent but collateral circulation was significantly less in group 2 than in group 1 (p <0.01, p <0.05, respectively). Cardiac output at rest was significantly lower in groups 1 and 2 than in healthy volunteers (4.52 ± 0.83 and 4.51 ± 0.84 vs 6.20 ± 0.83 L/min; p = 0.0014, p = 0.0003). Although RMBF at rest did not differ significantly among groups 1 and 2 and healthy volunteers (0.63 ± 0.20 vs 0.69 ± 0.13 and vs 0.77 ± 0.14 ml/min/g), RMBF after 5 minutes of exercise was significantly lower in group 2 than in group 1 and healthy volunteers (0.78 ± 0.11 vs 0.96 ± 0.20 and vs 1.20 ± 0.18 ml/min/g; p = 0.0289, p <0.0001). The number of regions of critical coronary artery disease was significantly greater in group 2 than in group 1 (4.0 ± 1.2 vs 2.1 ± 1.3, p = 0.0039). Our results suggest that the absence of QT interval shortening during exercise may indicate severe myocardial ischemia induced by exercise in patients with RBBB and coronary artery disease.