We investigated the role of ischemic hypoxia in the appearance of re-build-up phenomenon on electroencephalography in childhood moyamoya disease using 99mTc-hexamethyl propyleneamine oxime and single photon emission computed tomography (99mTc-HMPAO SPECT). In the case reported on, critical reduction of regional cerebral blood flow (rCBF) was observed during re-build-up phenomenon in the bilateral parieto-occipital area where acetazolamide testing revealed severe impairment of the perfusion reserve. On electroencephalography, the re-build-up phenomenon originated in these areas. In addition, re-build-up phenomenon developed into the entire hemisphere 30 s later, even where the severe ischemia was not observed. These results suggest that rCBF reduction induced by hyperventilation plays a critical role in the appearance of the re-build-up phenomenon, but other factors such as ischemic hypoxia after hyperventilation are also important in the development of this phenomenon.