Purpose: This study was performed to evaluate the outcome of encephalo-duro-arterio- synangiosis (EDAS) surgery with rest/acetazolamide Tc-99m HMPAO SPECT in moyamoya disease. Materials and Methods: Rest/acetazolamide subtraction SPECT with consecutive acquisition were done before and 2 months after 21 EDAS surgeries in 18 patients. Perfusion decrease was graded visually for 14 areas of each hemisphere as 0 (normal) to 3 (defectj using 4 point scoring system. Postoperative rest perfusion or perfusion reserve was compared with preoperative ones. Results: Among 294 areas of 21 hemispheres, rest perfusion abnormality was found in 97 areas of 15 hemispheres. Decrease of perfusion reserve was found in 146 areas of 18 hemispheres. Six hemispheres having normal rest perfusion and 12 of 15 hemispheres having rest perfusion abnormality showed reserve decrease. Three having rest perfusion defect did not change after acetazolamide in preoperative SPECT. After operation, 16 patients (89%) demonstrated clinical improvement. Fifteen among 18 hemispheres (83%) with decreased reserve improved. Rest perfusion abnormality improved in 6 among the 1.' hemispheres (40%). The areas having rest perfusion and/or reserve decrease improved in 8 : among 146 areas (60%). Decrease of reserve improved in 85% (68/80). However, areas without reserve decrease also improved in 29%(19/#66). The better was peoperative resi perfusion in involved areas or the more decreased vascular reserve, the rnore improved perfusion and reserve after operation. Conclusion: We conclude that assessment of perfusion and perfusion reserve using rest/acetazolamide brain perfusion SPECT predict the surgical outcome in patients with moyamoya disease. (Korean J Nucl Med 1998;32:314-24)