The coronary flow reserve was evaluated in 33 patients with stable effort angina and single vessel disease of the left anterior descending artery. We used a catheter-tip Doppler flow probe with injection of contrast media to the vessel in order to induce so-called reactive hyperemia. The reactive change was used as an index of the flow reserve of the coronary artery. In 15 patients out of 33, PTCA was performed and the change in the coronary flow reserve was evaluated. There was a good correlation between the coronary flow reserve and the exercise capacity in the treadmill ECG exercise test. As theoretically expected, the exercise capacity was determined by the coronary flow reserve. There was a poor correlation between the degree of stenosis shown in coronary arteriography and the coronary flow reserve. The degree of stenosis did not relate with the exercise capacity. The coronary flow reserve was increased with the procedure of PTCA. Though the patho-anatomic findings in coronary angiography give us important information to evaluate patients with ischemic heart disease, we concluded that these findings were still insufficient to allow us to estimate the flow reserve in the coronary artery. The exercise capacity did not relate well with the degree of stenosis.