Exercise Q, R, and S wave amplitude changes, called the QRS score, have been reported to be a marker of exercise-induced myocardial ischemia. Therefore, in this study, using the exercise QRS score, we sought to determine if slow coronary flow (SCF) phenomenon is associated with the exercise-induced myocardial ischemia. This retrospective study included 23 patients evaluated for suspected coronary artery disease and found to have SCF (group I) and 19 subjects with angiographically-defined significant coronary artery stenosis (group II). All study subjects underwent treadmill exercise testing using the modified Bruce protocol. For each subject the amplitude of the Q, R, and S waves in leads aVF and V5 was measured manually using calipers before and immediately after exercise. The QRS score was calculated by subtracting the Q, R, and S wave differences in leads aVF and V5. There was no difference between the two groups with respect to demographic properties. The peak heart rate achieved, baseline and peak systolic-diastolic blood pressure, exercise duration, and the metabolic equivalent values were similar in both groups. The maximum ST-segment depression ratio was significantly lower in patients with SCF than those of significant coronary stenosis (0.8 +/- 0.4 vs 1.3 +/- 0.5 P = 0.001, respectively). However, the exercise QRS score was found to be similar in both groups (3.3 +/- 2.3 vs 2.1 +/- 3.0 P = 0.2, respectively). The data suggest that SCF phenomenon may alone lead to myocardial ischemia even in the absence of obstructed major epicardial coronary arteries as detected by similar exercise QRS scores to those of significant coronary artery stenosis.