Uncertainty exists over the size of potential beneficial effects of medical treatments targeting hematoma growth in intracerebral hemorrhage (ICH). We report associations of hematoma growth parameters on clinical outcomes in the pilot phase, Intensive Blood Pressure Reduction in Acute Cerebral Hemorrhage Trial (INTERACT1) (ClinicalTrials.gov NCT00226096). In randomized patients with both baseline and 24-hour brain CT (n = 335), associations between measures of absolute and relative hematoma growth and 90-day poor outcomes of death and dependency (modified Rankin Scale score 3-5) were assessed in logistic regression models, with data reported as odds ratios (OR) and 95% confidence intervals (CI). A total of 10.7 mL (1 SD) increase in hematoma volume over 24 hours was strongly associated with poor outcome (adjusted OR 1.72, 95% CI 1.19-2.49; p = 0.004). An association was also evident for relative growth (adjusted OR 1.67, 95% 1.22-2.27; p = 0.001 for 1 SD increase). The analyses were adjusted for age, sex, achieved systolic blood pressure, elevated NIH Stroke Scale score (≥ 14), hematoma location, baseline hematoma volume, intraventricular extension, antithrombotic therapy, baseline glucose, time from ICH to baseline CT scan, and time from baseline to repeat CT scan. A 1 mL increase in hematoma growth was associated with a 5% (95% CI 2%-9%) higher risk of death or dependency. Medical treatments, such as rapid intensive blood pressure lowering, could achieve ∼2-4 mL absolute attenuation of hematoma growth. There is hope that this could translate into modest but still clinically worthwhile (∼10%-20% better chance) outcome from ICH.