Purpose: The aim of the study was to evaluate the hemodynamic changes after successful bypass surgery in patients with atherosclerotic stenosis in ICA using ^99mTc-ECD SPECT. Materials and Methods: Fourteen patients (M:F=8:6, mean age; 60¡¾9 years) who underwent STA-MCA anastomosis for unilateral atherosclerotic cerebrovascular disease were enrolled. ^99mTc-ECD basal/acetazolamide perfusion SPECT studies were performed before, 10 days and 6 months after bypass surgery. Perfusion reserve was defined as the % changes after acetazolamide over rest image. Regional cerebral blood flow and perfusion reserve were compared preoperative, early-postoperative and late-postoperative scans. Results: The mean resting perfusion and decrease in perfusion reserve in affected ICA territory on preoperative scan was 52.4¡¾3.5 and -7.9¡¾4.7%, respectively. The resting perfusion was significantly improved after surgery on early-postoperative scan (mean 53.7¡¾2.7) and late-postoperative scan (mean 53.3¡¾2.5) compared with preoperative images (p<0.05, respectively). Resting perfusion did not showed further improvement on late-postoperative scan compared with early-postoperative scan. The perfusion reserve was -3.7¡¾2.6% on early-postoperative scan, and -1.6¡¾2.3% on late-postoperative scan, which was significantly improved after surgery. Additionally, further improvement of perfusion reserved as observed on late-postoperative scan (p<0.05). While, in the unaffected ICA territory, no significant changes in the resting perfusion and perfusion reserve was observed. Conclusion: The improvement of resting perfusion and perfusion reserve in early-postoperative scan reflects the immediate restoration of the cerebral blood flow by bypass surgery. In contrasts, further improvement of perfusion reserve showing on late-postoperative scan may indicate a good collateral development after surgery, which may indicate good surgical outcome after surgery. (Nucl Med Mol Imaging 2006;40(6):293-301)