Exercise testing is commonly used to evaluate patients with coronary artery disease who have serious anatomic characteristics. To study the characteristic exercise test variables in patients with left main coronary artery disease, the computerized data base of the Hungarian Institute of Cardiology Exercise Test Laboratory was used. Among 2,378 patients who had undergone a supine bicycle exercise test and who had abnormal coronary angiographic results, 65 patients with significant (greater than 50 percent diameter narrowing) stenosis of the left main coronary artery were found. The 65 patients were subgrouped according to their previous history and other vessel involvement. Nine patients had isolated left main coronary artery disease and no myocardial infarction (group 1); 28 patients had left main coronary artery stenosis and another diseased vessel but no prior myocardial infarction (MI) (group 2); and 28 patients had left main coronary artery disease, another diseased vessel, and a prior MI (group 3). For comparison, the 27 patients selected to be the control group (group C) had no history of MI but had significant stenosis of both the left anterior descending and the left circumflex arteries. Exercise time, calculated oxygen consumption, maximal work load, time to ST depression, prevalence of ST segment depression, and maximal ST depression were similar in the groups. Maximal heart rate and double product were higher in group 1, but we could find no single variable or group of variables characteristic of left main coronary artery disease.