OBJECTIVES: To examine the epicardial U-wave changes recorded in intracoronary electrocardiograms (ECGs) during anterior or inferoposterior myocardial ischaemia and the corresponding changes in precordial ECGs recorded from the body surface in humans. METHODS: 40 patients undergoing coronary angioplasty of the left anterior descending (LAD) coronary artery (22 patients) or left circumflex (LC) artery (18 patients). Intracoronary and surface precordial ECGs were simultaneously recorded under baseline conditions and during angioplasty. RESULTS: Four different patterns of U-wave change were identified on the intracoronary ECG: change to positivity, no change, change to negativity, and biphasic change. The incidence of each pattern was similar in the two groups (LAD v LC groups, 23% v 39%; 23% v 17%; 41% v 44%; 13% v 0%, respectively). The intracoronary ECG was more sensitive than the surface ECG for detecting U-wave changes (intracoronary v surface ECG: LAD group, 77% v 55%; LC group, 83% v 28%). A study of the correlation between intracoronary and surface precordial ECGs showed that in patients who had U-wave changes in their intracoronary ECG (17 LAD and 15 LC patients) 65% of the LAD group but only 6% of the LC group had primary U-wave changes in the surface precordial ECG, and that 27% of the LC patients had reciprocal U-wave changes in the right to central precordial ECG. CONCLUSIONS: These results provide fundamental information for an understanding of the correlation between U-wave changes in the epicardial and surface pre-cordial ECGs during myocardial ischaemia in humans. As well as the primary U-wave changes seen in many of those with anterior myocardial ischaemia, some of those with posterior myocardial ischaemia had reciprocal U-wave changes in their surface precordial ECGs.