100 male patients with at least 75% obstruction of one or more coronary arteries were subjected to submaximal exercise tests. Among the 73 subjects who had positive tests, 92% of those with one-vessel obstruction had an ST depression of 1 mm, none having more than 2 mm; by contrast, 44% of those with three-vessel disease had an ST depression of more than 2 mm and only 27% an ST depression of 1 mm. Left-ventricular end-diastolic pressure exceeded 15 mm Hg in 86% of the patients who had an ST depression of more than 2 mm but only in 33% of those with a depression of 1mm; impaired contractility was found in 81% of the former and in 36% of the latter. 92% of those with one-vessel obstruction were able to perform work of 75 and 100 W/min while only 25% of those with three-vessel disease were able to perform the same amount of work. The peak exercise heart rate and systolic blood pressure also decreased with the increase in the number of affected vessels.