Coronary anatomy and the results of treadmill exercise testing were compared in two partially overlapping groups of patients. In the first part of the study, treadmill tests of 29 patients with significant (greater than or equal to 50%) left main stenosis were compared to those of 40 patients each with proximal 1-, 2-, and 3-vessel disease, respectively. Two distinct subgroups were identified among the patients with greater than or equal to 50% left main stenosis: Of those with greater than or equal to 70% left main stenosis all had a positive (ST decreases greater than or equal to 0.1 mV) stress test, 79% had a markedly positive (ST decreases greater than or equal to 0.2 mV) test and this group had the lowest exercise capacity. In patients with only moderate left main stenosis (50-70%) ST-segment depression was significantly less and occurred significantly later than in patients with severe left main stenosis and 3-vessel disease. Patients with 1-, and 2-vessel disease could be clearly separated from patients with either severe left main stenosis or 3-vessel disease, but not from those with only moderate left main stenosis. In the second part of the study the coronary anatomy of 62 patients with a markedly positive stress test was analysed. In these patients left main stenosis, 3-, 2-, and 1-vessel disease was observed in 10%, 50%, 23% and 16%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)