QT interval prolongation is discussed as a possible risk factor of dangerous arrhythmias. The aim of this study was to evaluate the influence of different interventional treatment of coronary artery disease in patients after myocardial infarction on QT interval duration. Two groups of patients were analyzed. The first group (G1) consisted of 37 patients in whom PTCA was performed; the second group (G2)--42 pts after surgical myocardial revascularisation (CABG). The 24-hour electrocardiography monitoring was performed in all the patients. Results were analyzed especially considering the average daily values of QT, QTp, QTc and RR duration. In patients after CABG we observed statistically significant prolongation of QTc interval duration what was not noticeable in patients after PTCA. The cardiosurgical procedure of coronary by-pass grafting in patients with coronary artery disease after the myocardial infarction in contrary of PTCA may lead to significant prolongation of QTc interval duration. This problem needs further investigation.