Background and Purpose-Stroke is the third leading cause of death and the leading cause of disability in the United States. Intracerebral hemorrhage and subarachnoid hemorrhage represent ≈20% of all stroke cases and have a mortality rate of 40% to 50%. Hypertension is an important risk factor for these subtypes of stroke. We sought to determine whether untreated hypertension carries a different risk from treated hypertension for hemorrhagic stroke. Methods-Cases of hemorrhagic stroke in the greater Cincinnati region were identified by screening all area hospital emergency rooms, radiology reports, and International Classification of Diseases 9 codes. Medical records were reviewed for risk factors and medication use. Cases of hemorrhagic stroke were approached for enrollment into the genetic sampling and interview arm. If subjects agreed, the case was matched by age, race, and gender to population-based controls. Results-Between May 1997 and December 2002, we recruited 549 cases of hemorrhagic stroke, of which 322 were intracerebral hemorrhage and 227 were subarachnoid hemorrhage. Untreated hypertension was found to be a significant risk factor for hemorrhagic stroke (odds ratio [OR]=3.5 [2.3 to 5.2]; P<0.0001) as was treated hypertension (OR=1.4 [1.0 to 1.9]; P=0.03). Insurance status of "self-pay" or Medicaid was a significant risk factor for untreated hypertension (OR=2.7 [1.6 to 4.4]). We estimate that 17% to 28% of hemorrhagic strokes among hypertensive patients would have been prevented if they had been on hypertension treatment. Conclusion-Untreated hypertension is highly prevalent and an important risk factor for hemorrhagic stroke. We estimate that among hypertensive subjects, approximately one fourth of hemorrhagic strokes would be prevented if all hypertensive subjects received treatment.