The signal-averaged electrocardiogram (SA-ECG) was studied in 148 patients undergoing myocardial revascularisation either by coronary bypass surgery (CBS) (64 cases) or transluminal angioplasty (PTCA) (84 cases). The investigation was performed before the procedure, at day 7 (D7) and after 3 months (D120). No difference was observed in the SA-ECG between the two groups before revascularisation. The CBS population was older, more symptomatic and had more severe lesions than the PTCA population. One hundred and thirty-nine patients were followed up until the end of the study protocol (CBS = 63; PTCA = 76). Two criteria of the SA-ECG were significantly modified after CBS: QRS duration (p < 0.05) and Under 40 (p < 0.01). No significant changes were observed after PTCA. In the patients with late potentials (LP) before revascularisation, the mean value of these criteria (Under 40 and Last 40) were significantly modified after CBS (U40 = 54.3 +/- 16 to 35.4 +/- 15; p < 0.01) (L40 = 11.9 +/- 4.7 to 26.1 +/- 24.3; p < 0.01). No changes in these criteria were observed after PTCA. The value of negativation of the criteria of LP for patients with two criteria of positivity was 71.1% after CBS compared with 25% after PTCA (not significant). These observations support the hypothesis of a favourable modification of the arrhythmogenic substrate after myocardial revascularisation, especially by CBS.