Regional cerebral blood flow was measured by the 133Xe inhalation method in 20 young patients with moyamoya disease and five young healthy volunteers. Most patients showed low values of mean hemispheric blood flow in both hemispheres. Regional cerebral blood flow was at a low value in the upper frontal region and at an almost average value in the posterotemporal and occipital regions, which was different from the "hyperfrontal" pattern in healthy volunteers. Regional cerebral blood flow was reduced evenly by hyperventilation. By 5% CO2 inhalation, regional cerebral blood flow was increased in the temporooccipital regions and was nearly unchanged or decreased in the frontal region.