A study has been made of the cerebral blood flow (CBF) in moyamoya disease from the perspective of hemispheric cerebral blood flow and regional CBF (rCBF). The material includes 21 children between the ages of 5 and 15 years with moyamoya disease, and 19 adult moyamoya cases--all of which had virtually no neurological symptoms at the time of the study. CBF was measured using the 133Xe intravenous injection method. Comparison was made with the measurements from 16 normal children and 14 normal adults. Study was also made of the relationship between the angiographic stage of the disease and the CBF. With the exception of the more elderly patients, CBF was found to be significantly lower in the moyamoya cases than in normal subjects of the same age group. In all age groups, the distribution of rCBF showed a dominant posterior distribution, dissimilar to the dominant anterior distribution found in the normals. Among the juvenile moyamoya cases, there was a tendency toward decreasing hemispheric blood flow together with advancing disease--as determined angiographically. Moreover, with advancing stages of the disease, there was a continuing transition from the normal pattern of frontal dominance to one of occipital dominance. This dominance of posterior rCBF is thought to be a characteristic feature of moyamoya disease.