To determine if patterns of ST depression or elevation during exercise testing provide reliable information about the location of an underlying coronary lesion, we studied 452 consecutive patients with one-vessel disease who underwent treadmill testing. Exercise ST changes were classified as elevation or depression and by lead groups involved. The ST depression occurred most commonly in leads V5 or V6 regardless of which coronary artery was involved. In contrast, anterior ST elevation indicated left anterior descending coronary disease in 93% of cases, and inferior ST elevation indicated a lesion in or proximal to the posterior descending artery in 86% of cases. Furthermore, anterior ST elevation in leads without diagnostic Q waves usually indicated a high-grade, often proximal, left anterior descending stenosis, whereas anterior ST elevation in leads with Q waves usually indicated a totally occluded left anterior descending coronary artery. Thus, ST elevation during exercise testing, although uncommon, is a reliable guide to the underlying coronary lesion, whereas ST depression is not.