Background: Arterial hypertension is common in the first 24 hours after acute intracerebral hemorrhage (ICH). Although increased blood pressure usually declines to baseline values within several days, the appropriate treatment during the acute period has bemained controversial. Arguments against treatment of hypertension in patients with acute ICH are based primarily on the concern that reducing arterial blood pressure will reduce cerebral blood flow (CBF). The authors undertook this study to provide further information on the changes in whole-brain and periclor regional CBF that occur with pharmacologic reductions in mean arterial pressure (MAP) in patients with acute ICH. Methods: Fourteen patients with acute supratentorial ICH 1 to 45 mL in size were studied 6 to 22 hours after onset. CBF was measured with PET and 15O-water. After completion of the first CBF measurement, patients were randomized to receive either nicardipine or labetalol to reduce MAP by 15%, and the CBF study was repeated. Results: MAP was lowered by -16.7 ± 5.4% from 143 ± 10 to 119 ± 11 mm Hg. There was no significant change in either global CBF or periclor CBF. Calculation of the 95% CI demonstrated that there is less than a 5% chance that global or periclor CBF fell by more than -2.7 mL 100 g-1 min-1. Conclusion: In patients with small- to medium-cized acute ICH, autoregulation of CBF was preserved with arterial blood pressure reductions in the range studied.