Double product (DP, heart rate × systolic blood pressure) during an incremental exercise test has been known to start to increase steeply at a workload, i. e. double product break point (DPBP), which corresponds to the blood lactate threshold. The study was to investigate the relationship between DP transition and ST segment depression in electrocardiogram during exercise. Thirty-one patients of angina pectoris of effort and 140 elderly persons performed a continuous incremental exercise test using a stationary bicycle ergometer. During the entire test, HR and blood pressure were measured every 15 seconds and an electrocardiogram was recorded continuously. DPBP was then calculated. In ten of the 31 patients, ST level depression above -0.1 mV with a typical ischemic form in lead V 5 was observed during the test. However, workload at the DPBP was significantly lower than that of the ischemic threshold on the electrocardiogram (i. e. -0.1mV of ST depression) in relation to work load (46+/-16 vs. 78+/-20 watts), HR (96+/-13 vs. 117+/-13 bpm), SBP (160+ /-20 vs. 199+/-31 mmHg) and DP (15400+/-3400 vs. 23400+/-4900 bpm × mmHg) . In elderly persons, DPBP could be determined without the ischemic ST depression in 96% of 327 tests. This study indicated that the DPBP would be an objective index of exercise intensity with lower risk for exercise prescription in cardiac patients and elderly persons.