PURPOSE Quantitative measurement of regional cerebral blood flow (rCBF) needs special equipment or invasive blood sampling. Therefore, with a popularly used rotating gamma camera, we tried to establish a convenient modality of qualitative evaluation of hemodynamics in patients with atherosclerotic large-artery disease, according to the distribution pattern of isotope-uptake on SPECT image. PATIENTS AND METHODS Forty-one patients with atherosclerotic large-artery disease [9 internal carotid artery (ICA) stenosis, 17 ICA occlusion, 8 middle cerebral artery (MCA) stenosis, and 7 MCA occlusion] and 32 control patients documented as having small cerebral infarction without atherosclerotic large-artery disease were investigated. Using the rotating gamma camera, they underwent 123I-IMP SPECT at baseline and after acetazolamide loading. According to the distribution pattern of low isotope-uptake, early images were classified into four types: Type I: no low uptake, Type II: low uptake in a single watershed, Type III: low uptake in two watersheds with or without a partial MCA territory, Type IV: low uptake in a whole MCA territory. RESULT In patients with large-artery disease, baseline images presented Type I in 12 cases, Type II in 16, Type III in 9, Type IV in 4, and loading images Type I in 3, Type II in 2, Type III in 8, Type IV in 28. Of 37 patients except for 4 belonging to Type IV at baseline, 32 (86.5%) had a more extensive low isotope-uptake area after acetazolamide loading than at baseline. In control patients, baseline images presented Type I in 25, Type II in 7, and the Type III & IV in 0, and the Type after loading was the same as the Type at baseline. CONCLUSION We established the qualitative evaluation of 123I-IMP SPECT image by detecting the "extent" of the ischemic region, taking notice of watershed. The pathognomonic findings on SPECT image of atherosclerotic large-artery disease showed that low isotope-uptake area at baseline extended into two watersheds or MCA territory by acetazolamide loading. The modality of qualitative evaluation based on these pathognomic findings of SPECT image may be useful for easily assessing patients with atherosclerotic large-artery disease using a rotating gamma camera.