To evaluate the clinical significance of asymptomatic ischemic heart disease, exercise electrocardiography and stress myocardial scintigraphy were performed. These were correlated with symptoms during exercise tests and histories of myocardial infarction (MI). The study subjects consisted of 70 patients with coronary artery disease, including 34 with MI, and 36 without MI but with angina pectoris. Stress tests were performed using bicycle ergometer under electrocardiographic monitoring throughout the test. Transient myocardial ischemia was confirmed by perfusion defects on thallium myocardial imaging demonstrated immediately after exercise, but not 3 hours after the stress test. Asymptomatic ST depression was observed in 18 of 34 patients with MI (53%) and in 21 of the 36 patients with angina (58%); however, transient myocardial perfusion defects were confirmed in 61% of the patients with MI (11 of 18 patients), but in only 33% of those with angina (7 of 21 patients). The difference was statistically significant (p less than 0.05). It was suggested that there are some differences in the clinical significance of asymptomatic ST depression between the patients with MI and those without MI but with angina pectoris.