Fifty patients underwent a 24-hour Holter system recording immediately after successful coronary angioplasty. Only those patients who had been "successfully" dilated and who, during the following 2 days, had remained totally symptomless and without changes in standard ECG were selected. Arrhythmias occurred in 18 patients: 12 had supraventricular arrhythmia, including 3 prolonged attacks of tachyarrhythmia due to atrial fibrillation; 6 had ventricular arrhythmia, with numerous extrasystoles in 5 cases and bursts of ventricular tachycardia in 1 case. Changes in ventricular repolarization were recorded as: (1) isolated T-wave modification (11 patients), and (2) ST-segment depression (11 patients) reaching or exceeding 2 mn in 5 cases and lasting from 4 to 33 minutes. These silent and transient electrical abnormalities were observed mostly during the 12 hours which followed transluminal angioplasty, and particularly after dilatation of the right coronary artery. The physiopathological mechanisms of these changes are uncertain, but their occurrence has no influence on mid-term results, i.e. the follow-up coronary arteriography at 6 months.