The standard 12-lead electrocardiogram (ECG) fails to detect ST-segment elevation in patients with posterior wall acute myocardial ischemia. However, additional posterior leads V(7-9) provide limited additional diagnostic information to the standard 12-lead ECG when an ischemic criterion of 1-mm ST elevation is used. No study is available to delineate the ischemic criteria in the posterior electrocardiographic leads. Continuous 15-lead ECGs (standard 12 lead + V(7-9)) were recorded in 53 subjects undergoing elective left circumflex coronary angioplasty (posterior ischemia model). ST amplitudes (J + 60 ms) at preangioplasty baseline were subtracted from maximal ST amplitudes during balloon occlusion to create a positive or negative change score (DeltaST) for each of the 15 leads. During 53 left circumflex occlusions, 26 subjects (49%) had DeltaST elevation of > or = 1 mm and 24 subjects (45%) had DeltaST elevation ranging from 0.5 to 0.95 mm in > or = 1 posterior leads. Five subjects (9%) had DeltaST elevation of > or = 1 mm in the posterior leads without DeltaST elevation anywhere in any of the 12 leads. The sensitivity in detecting myocardial ischemia using 15-lead ECGs (58%) was not statistically different from the standard 12-lead ECG (49%) (p = 0.06). Adjusting the ischemic criterion from 1 to 0.5 mm in V(7-9) significantly improved the sensitivity from 49% in the 12-lead ECG to 94% in the 15-lead ECG (p = 0.000). In addition, 12 subjects (23%) had posterior ST-segment elevation without anterior ST-segment depression. Thus, posterior leads V(7-9) contribute significant additional diagnostic information above and beyond the standard 12-lead ECG only when a new ischemic criterion of 0.5 mm instead of 1 mm ST elevation is applied to the posterior leads.