The aim of this study was to evaluate the usefulness of dobutamine echocardiography and quantitative thallium-201 single photon emission computed tomography myocardial scintigraphy with reinjection in the detection of viable myocardium in patients with coronary artery disease and reduced left ventricular function, which will improve after aortocoronary bypass surgery. Forty-eight patients (47 men, aged 61 ± 6 years) with angiographically documented reduced left ventricular function (ejection fraction 35 ± 14, 63% with chronic transmural myocardial infarction) were examined by dobutamine two-dimensional echocardiography (before and during low dosage), ²⁰¹Tl , and gated radionuclide ventriculography before and 3 ± 2 months after aortocoronary bypass surgery. Four of 55 areas classified viable before operation were revascularized inadequately and discarded. Global left ventricular ejection fraction at rest rose from 35% ± 14% before operation to 40% ± 13% (p < 0.05) after operation. Stress-induced perfusion defects involved 40% ± 19% of the left ventricle circumference after stress and showed a significant reduction of size to 23% ± 14% (p < 0.01) at rest, 4 hours later, and after reinjection. This value fell to 16% ± 12% (p < 0.05) 3 months after aortocoronary bypass surgery. We conclude that both dobutamine echocardiography (sensitivity 95%, specificity 80%, positive predictive value 87%, negative predictive value 88%) and ²⁰¹Tl studies (sensitivity 87%, specificity 98%, positive predictive value 97%, negative predictive value 93%) are suitable and comparable accurate methods for predicting improvement in systolic function 3 months after revascularization in a selected population with a high prevalence of viable but hypokinetic or akinetic myocardium. (Am Heart J 1998;136:463-75.)