The purpose of this study was to clarify the clinical and prognostic significance of silent myocardial ischemia (SMI) in patients with proven ischemic heart disease using ambulatory electrocardiographic monitoring (AEM). A total of two hundreds and ninety one patients, including 131 patients with stable effort angina (EA), 125 with previous myocardial infarction (MI), 21 with variant angina (VA), and 14 with unstable angina (UA) were examined. In 141 (EA 65, MI 41, VA 21, UA 14) of these 291 patients, we investigated the frequency and characteristics of SMI. During daily activities, 47% (125/268), 73% (99/136), 71% (140/196), and 69% (102/147) of all transient ST-segment deviations observed was asymptomatic in patients with EA, MI, VA, and UA, respectively. When patients exhibiting only symptomatic episodes were compared with those exhibiting only asymptomatic ones, asymptomatic episodes tended to be associated with the greater duration and magnitude of ST-segment deviation. On the other hand, in patients exhibiting both symptomatic and asymptomatic episodes, the duration and magnitude of ST-segment deviation were greater for symptomatic than asymptomatic episodes. In addition, we also investigated the relation between the result of AEM after standard medical therapy and long-term prognosis in 208 (EA 99, MI 109) of 291 patients. In MI, during the mean follow-up period of 41.2 months, cumulative cardiac event rate was significantly higher in patients with only asymptomatic episodes (SMI group) as well as patients with symptomatic episodes (angina pectoris, AP group) than in patients without any ischemic episodes (Control group). There was no statistically significant difference in prognosis between AP group and SMI group in MI, or among the three groups in EA. These results lead us to conclude that: (1) SMI is observed frequently in patients with ischemic heart disease during daily activities. (2) Asymptomatic episodes are not necessarily less severe in myocadial ischemia than symptomatic episodes. (3) The presence of SMI, as well as angina pectoris, may identify a high-risk group for the development of subsequent unfavorable outcomes while on standard medical therapy.