Background: This study examined the prognostic value of adenosine single-photon emission computed tomographic thallium imaging in medically treated patients with angiographic evidence of coronary artery disease (CAD). Methods and Results: Patients who underwent coronary revascularization within 3 months of this study were excluded. There were 177 patients aged 64±11 years; 74 had one-vessel, 57 had two-vessel, and 46 had three-vessel CAD (>-50% diameter stenosis). During a mean follow-up of 22±13 months, there were 14 events (cardiac death or nonfatal myocardial infarction). Cox survival analysis with important clinical, catheterization, and scintigraphic variables identified the size of perfusion abnormality as the strongest predictor of events (x2=9). Life-table analysis showed that patients with perfusion defects of 15% or greater of the myocardium had a worse prognosis than had patients with no or smaller defects (Mantel-Cox statistic =13;p<0.001). Conclusions: Thus adenosine single-photo emission computed tomographic thallium imaging provides important prognostic data in medically treated patients with CAD. The extent of thallium abnormality is the most important predictor of events. © 1994 American Society of Nuclear Cardiology.