To elucidate the pathological aspects of silent myocardial ischemia, we studied 24 patients with ischemic heart disease who had culprit lesions in the left anterior descending artery (LAD). We determined the presence of myocardial ischemia and measured coronary wedge pressures (CWP; mmHg) and collateral circulation and ST deviation on the ECG (intracoronary ECG: ic-ECG, and surface ECG; mm) after balloon inflation during PTCA intervention. The study subjects included 9 with exertional angina, 10 with post-infarction angina, and 5 with Cohn type II angina. During 78 balloon inflations, the group of ischemic symptoms (Group S) occurred in 40% of all cases, the group without ischemic symptoms (Group A) constituted 45%, and the Cohn type II specific for ischemic symptoms accounted for 15%. The relationship between CWP (X-axis) and ST deviation (Y-axis) of ic-ECG was: Y = -0.46X + 20.19 (r = -0.59; p < 0.01), and the relationship between CWP and ST deviation of the surface ECG was: Y = -0.12X + 6.58 (r = -0.42; p < 0.01). Thus, a negative correlation was confirmed between them. Furthermore, similar results were obtained for Groups S and A. Based on this relationship, the pain threshold was estimated. In Group S, CWP exceeded 34 mmHg, i.e., ischemia was expected to be mild because of good collateral circulation, but an average ST deviation accompanying ischemic symptoms was observed. However, in Group A, CWP was less than 24 mmHg, i.e., ischemia was expected to be severe due to poor collateral circulation, but an average ST deviation lacking ischemic symptoms was observed. Comparison of these results showed that the pain threshold observed from the ST deviation of ic-ECG was 6.0-6.5 mm and that of the surface ECG was 2.6-2.8 mm. From these threshold values, the ST deviations during 12 balloon inflations in the Cohn type II were evaluated. Because 100% of ic-ECG and 75% of surface ECG exhibited values exceeding the threshold values, it was concluded that the cause of the Cohn type II was an increase of the pain threshold. ST deviations of the ic-ECG for Group S and the Cohn type II were 12.0 +/- 6.7 and 9.8 +/- 2.7 mm, respectively, and ST deviations of the surface ECG were 4.7 +/- 2.4 and 3.5 +/- 1.7 mm, respectively. Since there were no significant differences between Group S and the Cohn type II, it was concluded that the ischemic degree of the Cohn type II was approximately the same as that of Group S.