To continuously record an intracoronary ECG during the crucial phase of percutaneous transluminal coronary angioplasty, the coronary guide wire was connected to an ECG recorder. In 25 patients the intracoronary ECG was recorded simultaneously with standard leads I, II and III during balloon occlusion of a coronary artery for 30-60 sec. The wire serving as electrode was positioned in the distal third of the coronary artery to be dilated, thus reflecting changes in the pertinent area of the myocardium. This was the left anterior descending coronary artery in 19 patients, the left circumflex coronary artery in 4 patients, and the right coronary artery in 2 patients. Electrocardiographic signs of ischemia were observed in 16 patients (62%). They were present in both the intracoronary ECG and at least 1 of the standard leads I, II or III in 11 patients, in the intracoronary ECG exclusively in 4 patients, and in the standard lead I exclusively in 1 patient. Sensitivity and specificity as an indicator for chest pain during balloon occlusion were 88% and 89% respectively for the intracoronary ECG and 63% and 78% respectively for the standard leads I, II and III. Sensitivity and specificity as an indicator for poor collateralization (coronary wedge pressure less than or equal to 25 mm Hg) were 100% and 69% respectively for the intracoronary ECG, and 60% and 62% respectively for the standard leads, I, II and III.(ABSTRACT TRUNCATED AT 250 WORDS)