This study was designed to evaluate whether treadmill stress testing would facilitate selection of patients with advanced coronary artery disease and, specifically, whether markedly abnormal ischemic responses could be used as indicators of severity of disease. Among 59 consecutive patients with documented coronary artery disease having both maximal treadmill testing and coronary angiographic studies, 15 (group 1) had normal responses to exercise, 18 (group 2) showed 1 to 2.9 mm "ischemic" (flat or downward-sloping) ST-segment depression, and 26 (group 3) demonstrated marked (or equal to 3 mm) ischemic responses. Group 3 had statistically significant higher incidences of triple-vessel disease (18/26; 69 percent) and proximal lesions of the left anterior descending coronary artery (24/26; 92 percent), compared with group 1 (2/15 and 10/15, respectively) and group 2 (6/18 and 12/18, respectively). Group 3 also manifested more extensive disease than groups 1 and 2 (judged by scoring system of Friesinger et al), with a score of 11 or more in 18 of 26 patients. We conclude that marked depth of "ischemic" ST-segment depression aids in identifying that subgroup of the coronary population with severe coronary artery disease and, therefore, serves as a useful means of culling out patients with a potentially serious prognosis who might benefit from intensive diagnostic or therapeutic interventions.