A new exercise electrocardiography test has been examined in patients with angina pectoris; the rate of development of ST segment depression with respect to increases in heart rate during exercise on a bicycle ergometer was measured during exercise using 13 leads in 64 patients. The steepest slope of regression lines relating displacement of ST segment to increases in heart rate (maximal ST/HR slope) was used as an index of myocardial ischaemia and was compared with the results of coronary angiography which were determined by the radiologists and the cardiologist responsible for the management of these patients. The maximal ST/HR slope could be derived consistently from a linear ST/HR relationship (r greater than 0.95) only in 50 patients with significant coronary artery disease (greater than or equal to 75% liminal narrowing). The ranges of the maximal ST/HR slope in 17 patients with single-vessel disease, in 18 patients with double-vessel disease and in 15 patients with triple-vessel disease were different from each other and the differences between the means were statistically significant (P less than 0.0005). The maximal ST/HR slopes for the patients with single-vessel disease were also different from those in the 14 patients in whom significant coronary disease could not be demonstrated. In contrast, the criteria of heart rate at which ST segment depression began, maximum ST segment depression, rate-pressure product and heart rate attained at the end of the test showed an overlap between the groups of the patients studied; using the usual exercise test criteria in the same population, there were three false negative results, one false positive result and the results in eight of the patients were indeterminate. It is concluded that the maximal ST/HR slope, used as an index of myocardial ischaemia, reliably predicted the presence and severity of coronary artery disease, as determined by coronary arteriography in each of the patients with anginal pain.