Cerebral autoregulation is impaired in Himalayan high-altitude residents who live above 4200 m. This study was undertaken to determine the altitude at which this impairment of autoregulation occurs. A second aim of the study was to test the hypothesis that administration of oxygen can reverse this impairment in autoregulation at high altitudes. In four groups of 10 Himalayan high-altitude dwellers residing at 1330, 2650, 3440 and 4243 m, arterial oxygenation (Sao₂), blood pressure and middle cerebral artery blood velocity were monitored during infusion of phenylephrine to determine static cerebral autoregulation. Based on these measurements the cerebral autoregulation index (AI) was calculated. Normally, AI is between zero and 1. AI=0 implies absent autoregulation and AI=1 implies intact autoregulation. At 1330 m (Sao₂=97%), 2650 m (Sao₂=96%) and 3440 m (Sao₂=93%), AI values were respectively 0.63±0.27 (mean±SD) , 0.57±0.22 and 0.57±0.15. At 4243 m (Sao₂=88%), AI was 0.22±0.18 (P<0.0005, compared with AI at the lower altitudes), and increased to 0.49±0.23 (P=0.008, paired t test) when oxygen was administered (Sao₂=98%). In conclusion, high-altitude residents living at 4243 m have almost total loss of cerebral autoregulation, which improved during oxygen administration. Those people living at 3440 m and lower have still functioning cerebral autoregulation. This study showed that the altitude region between 3440 and 4243 m, marked by Sao₂ in the high-altitude dwellers of 93% and 88%, is a transitional zone, above which cerebral autoregulation becomes critically impaired.