The pilot phase of the Intensive Blood Pressure Reduction in Acute Cerebral Haemorrhage Trial (INTERACT) showed that rapid blood pressure (BP) lowering can attenuate hematoma growth in acute intracerebral hemorrhage. We sought to define the systolic BP level associated with greatest attenuation of hematoma growth. INTERACT included 404 patients with computed tomographic–confirmed intracerebral hemorrhage, elevated systolic BP (150 to 220 mm Hg), and capacity to commence BP lowering treatment within 6 hours of onset. Computed tomography was done at baseline and at 24 hours using standardized techniques, with digital images analyzed centrally, blinded to clinical data. Associations of baseline and achieved on-treatment (mean during the first 24 hours) systolic BP levels with the primary outcome of increase in hematoma volume were explored. There were 346 patients with duplicate computed tomographic scans. There was no significant association between baseline systolic BP levels and either the absolute or proportional growth in hematoma volume (P trend=0.26 and 0.12, respectively). By contrast, achieved on-treatment systolic BP levels in the first 24 hours were clearly associated with both absolute and proportional hematoma growth (both P trend=0.03). Maximum reduction in hematoma growth occurred in the one third of participants with the lowest on-treatment systolic BP levels (median: 135 mm Hg). Intensive BP reduction to systolic levels between 130 and 140 mm Hg is likely to provide the maximum protection against hematoma growth after intracerebral hemorrhage.