The objective of this study was to examine the prevalence and pathogenesis of silent myocardial ischemia during exercise following myocardial infarction. Exercise-induced myocardial ischemia was assessed by ²⁰¹Tl-SPECT (single photon emission computed tomography) 4.5 weeks after acute myocardial infarction in 229 patients. Exercise-induced myocardial ischemia occurred in 109 patients (48%), and 72 (32%) had silent ischemia. Although the prevalence of multivessel coronary artery disease was similar between patients with silent and symptomatic ischemia, the size of reversible myocardial ischemia was larger in patients with symptomatic ischemia than in those with silent ischemia (21.3±3.0% vs 13.2±1.9% of LV, p<0.05). The incidence of reversible ischemia remote from the infarct area was higher in patients with symptomatic ischemia than in those with silent ischemia (30% vs 17%, p<0.10). The cause of silent ischemia after myocardial infarction may be closely related to the smaller size of reversible myocardial ischemia. Ischemia remote from, or adjacent to, the infarct area could be a factor in determining the presence or absence of pain.