Background : Off pump coronary artery bypass graft(OPCAB) is one of the current less invasive coronary bypass surgery. We studied flow reserve of myocardium between OPCAB and conventional coronary artery bypass graft(CCAB) using Tl-201 perfusion SPECT at 3-12 months after bypass surgery. Methods : Between November, 1992 and December, 2001, a total of consecutive 249 patients underwent CABG by the same cardiac surgeon. 152 patients of them were recruited from 1997 to 2001 (OPCAB, n=100, M/F=76/24, 62짹8 years; CCAB, n=52, M/F=36/16, 65짹7years). All patients underwent pharmacological stress Tl-201 perfusion SPECT using dipyridamole or adenosine at 3-12 months after bypass surgery(OPCAB: 178짹47days; CCAB: 164짹57days). Myocardial perfusion was analysed semiquantitatively using a five-point scoring system in a 20-segment model (0=normal to 4=absent uptake). Summed stress(SSS), rest(SRS), and difference score(SDS) of whole myocardium as well as average scores of individual wall(anterior, septal, lateral and inferior) were compared by student t-test as well as by repeated measure ANOVA with Bonferoni correction. Results : SSS, SRS, SDS of OPCAB group were 6.86짹0.72, 3.95짹0.57, 2.91짹 0.47,respectively, which were not significantly different from those of CCAB group (7.17짹0.92, 3.75짹0.73, 3.42짹0.74). However, lateral wall demonstrated lower scores in OPCAB group (average stress scores:0.18짹0.04 vs 0.41짹0.08,p=0.015; average rest score:0.12짹0.03 vs 0.20짹0.06,p=0.168; average difference score:0.06짹0.02 vs 0.21짹0.06,p=0.031). Septal wall resulted in higher scores in OPCAB group(average stress scores:0.33짹0.06 vs 0.12짹0.03, p=0.003; average rest score:0.18짹0.04 vs 0.07짹0.03, p=0.037; average difference score:0.14짹0.04 vs 0.04짹0.02, p=0.030). Scores of anterior and inferior wall were not different between the two groups. Conclusions : OPCAB shows better surgical outcome in respect of myocardial perfusion reserve. It must be the combined effect of arterial grafting that gave advantage over CCAB in lateral wall.