The present study compared the clinical prediction of the effect of coronary artery bypass grafting (CABG) on coronary blood flow and left ventricular ejection fraction (LVEF) with changes in gated myocardial perfusion scintigraphy. A prospective group of 92 patients underwent myocardial perfusion scintigraphy before and 6 months after CABG, the results being kept secret from the surgeon. Based on clinical and angiographic findings, the surgeons filled in a questionnaire indicating the predicted changes in coronary blood flow in each of the three coronary artery territories and in the LVEF. Symptomatic improvement was present in nearly all the patients. Following CABG, the perfusion defects were reduced in around two-thirds and normalized in one-third of the territories clinically predicted to improve. Improved perfusion for territories not predicted to improve was slightly lower, and correlations between predicted and observed regional changes in coronary blood flow and perfusion defects were poor. LVEF increased (by over five ejection fraction units) in almost half of the patients, but with no correlation between the predicted and the observed changes. Based on clinical and angiographic findings, the marked improvements after CABG in cardiac perfusion and function are poorly predicted. Keywords: Coronary artery bypass grafting; Coronary artery disease; Myocardial perfusion scintigraphy.