This study examined the left ventricular perfusion and EF by using simultaneous SPECT and first-pass radionuclide angiography with technetium 99m sestamibi in 95 patients after uncomplicated coronary artery bypass grafting. The patients were divided into those with normal EF and no previous myocardial infarction before surgery (group 1, n = 57), and those with abnormal EF or infarction (group 2, n = 38). The SPECT images were normal in 37 patients in group 1 and in 6 patients in group 2 (p < 0.0001). The patients with normal SPECT images had a higher EF after surgery than those with abnormal images (65% ± 10% vs 50% ± 14%, p < 0.0001) and was higher in group 1 than in group 2 (64% ± 8% vs 46% ± 16%, p < 0.0001). There was a significant correlation between the EF and the extent of perfusion abnormality (r = -0.44, p < 0.0001). The patients with normal SPECT images could not be separated from those with abnormal images based on peak CK, CK-MB, and the electrocardiographic changes. Of the 69 patients with postoperative EF ≥50%, the perfusion pattern was normal in 41 and abnormal in 28; of the 26 patients with EF <50%, 24 had abnormal SPECT (p < 0.003). There was no significant change in mean EF after surgery (55% ± 14% before vs 56% ± 15% after). Thus simultaneous assessment of left ventricular perfusion and function after coronary artery bypass grafting showed that an abnormal perfusion pattern may exist despite a normal EF. These patients could not be predicted by enzymes or electrocardiographic changes. © 1993.