Sickle cell disease (SCD) is associated with a high incidence of ischemic stroke. SCD is characterized by hemolytic anemia, resulting in reduced nitric oxide-bioavailability, and by impaired cerebrovascular hemodynamics. Cerebrovascular CO2 responsiveness is nitric oxide dependent and has been related to an increased stroke risk in microvascular diseases. We questioned whether cerebrovascular CO2 responsiveness is impaired in SCD and related to hemolytic anemia. Transcranial Doppler-determined mean cerebral blood flow velocity (Vmean), near-infrared spectroscopy-determined cerebral oxygenation, and end-tidal CO2 tension were monitored during normocapnia and hypercapnia in 23 patients and 16 control subjects. Cerebrovascular CO2 responsiveness was quantified as $Δ$% Vmean and $Δ$$μ$mol/L cerebral oxyhemoglobin, deoxyhemoglobin, and total hemoglobin per mm Hg change in end-tidal CO 2 tension. Both ways of measurements revealed lower cerebrovascular CO2 responsiveness in SCD patients versus controls (Vmean, 3.7, 3.1-4.7 vs 5.9, 4.6-6.7 $Δ$% Vmean per mm Hg, P < .001; oxyhemoglobin, 0.36, 0.14-0.82 vs 0.78, 0.61-1.22 $Δ$$μ$mol/L per mm Hg, P = .025; deoxyhemoglobin, 0.35, 0.14-0.67 vs 0.58, 0.41-0.86 $Δ$$μ$mol/L per mm Hg, P = .033; total-hemoglobin, 0.13, 0.02-0.18 vs 0.23, 0.13-0.38 $Δ$$μ$mol/L per mm Hg, P = .038). Cerebrovascular CO2 responsiveness was not related to markers of hemolytic anemia. In SCD patients, impaired cerebrovascular CO2 responsiveness reflects reduced cerebrovascular reserve capacity, which may play a role in pathophysiology of stroke. © 2009 by The American Society of Hematology.