In an attempt to investigate which of the ST-segment parameters, e.g., ST-segment level, ST-segment slope, ST-segment index and ST-segment intergral obtained by mannual drawing can most sensitively indicate quantitatively the extent of exertional myocardial ischemia in patients with angina pectoris, 26 patients with angina pectoris underwent exrecise stress testing, using a bicycle ergometer and thallium-201 myocardial perfusion scintigraphy(Thallium-201 MPS). The exrecise was terminated either when symptoms or signs indicative of myocardial ischemia developed or when the heart rate reached the predicted maximal heart rate during exrecise. The myocardial ischemia region was quantitated by Thallium-201 MPS. The number of myocardial ischemia segments was similar in patients who had positive exrecise test(4.9+/-1.9) and in those who had negative result(3.7+/-2.2). However, it was greater in patients who showed pseudonormalization of the T wave in exercise ECG(6.3+/-2.6) than in those with negative exercise test(3.7+/-2.2)(P<0.05). The extent of myocardial ischemia region expressed by the number of myocardial ischemia segments correlates best with the ST-segment index(r=-0.83) among ST-segment slop(r=-0.60), ST-segment intergral(r=-0.59) and ST-segment(r=0.50). These data suggest that the ST-segment index is the most reliable indicator to reveal the extent of exertional myocardial ischemia among the ST-segment parameters and that pseudonormalization of T wave in exercise ECG is a finding indicative of exertional myocaridial ischemia in patients with angina pectoris.