BACKGROUND Because of limited surgical field, minimally invasive coronary artery bypass grafting (MIDCAB) requires anastomosis to the distal portion of the left anterior descending artery (LAD) of the left internal thoracic artery (LITA) with the heart beating. Though the diameters of these arteries are very small, it is unknown whether blood flow sufficient for the LAD territory is obtained by bypass grafting. METHOD Eight patients with single-vessel disease of the LAD underwent MIDCAB with the LITA to the LAD and we evaluate the perfusion and function in the LAD territory by quantitative ECG-gated SPECT (QGS) with 99m-technetium sestamibi (MIBI) before and after operation. RESULT The intraoperatively measured diameters of the LITA and LAD at the site of anastomosis were 1.1+/-0.2 mm and 1.3+/-0.4 mm, respectively. The percentage increases in end-diastolic perfusion, regional ejection fraction and regional wall thickening in the anteroseptal area after MIDCAB were 136.3+/-11.7(p=0.071), 148.4+/-6.6(p=0.007) and 133.0+/-5.6(p=0.029), respectively (paired t-test, mean +/- SD %). Stress-rest MIBI SPECT indicated no ischemia in anteroseptal wall. CONCLUSION The MIDCAB technique thus appeared to improve perfusion and function in the LAD territory despite bypass to the distal LAD, and ECG-gated MIBI SPECT using QGS software was very useful for evaluating the quality of anastomosis after MIDCAB.