This study was conducted to clarify the clinical significance of negative U waves in the precordial leads during anterior wall acute myocardial infarction (AMI). In all, 141 patients with first anterior wall AMI (< or = 6 hours) were classified into 2 groups according to the presence (group A, n = 31) or absence (group B, n = 110) of negative U waves in the precordial leads on the admission electrocardiogram (ECG). The number of leads showing ST elevation > or = 1 mm on the admission ECG was smaller in group A than in group B (5.2 +/- 1.3 vs 6.2 +/- 1.7, p < 0.01). Emergent coronary arteriography revealed that group A had a higher incidence of good collateral circulation than group B (39% vs 19%, p < 0.05). Peak creatine kinase activity was lower in group A than in group B (1,708 +/- 1,271 vs 2,735 +/- 1,865 IU/L, p < 0.01). The number of abnormal Q waves on the predischarge ECG was smaller in group A (2.0 +/- 1.5 vs 3.4 +/- 2.0, p < 0.01). Group A had a greater left ventricular ejection fraction and better regional wall motion in the anterobasal, anterolateral, and apical regions in the chronic phase than group B. In conclusion, patients with anterior wall AMI having negative U waves in the precordial leads on admission had a relatively smaller mass of necrotic myocardium than those without the waves. Therefore, negative U waves during anterior wall AMI may be a useful marker for identifying patients with smaller infarction partly due to better collateral circulation.