OBJECTIVE To investigate the impact of the adjustment of initial intracerebral hemorrhage (ICH) volume by onset-to-imaging time (ultraearly hematoma growth [uHG]) on further hematoma enlargement and outcome in patients with acute ICH. METHODS We studied 133 patients with acute (<6 hours) supratentorial ICH. Patients underwent baseline and 24-hour CT scans for ICH volume measurement, and a CT angiography (CTA) for the detection of the spot sign. We defined uHG as the relation between baseline ICH volume/onset-to-imaging time, hematoma growth (HG) as hematoma enlargement >33% or >6 mL at 24 hours, early neurologic deterioration (END) as increase ≥4 points in the NIH Stroke Scale score or death at 24 hours, and poor long-term outcome as modified Rankin Scale score >2 at 3 months. RESULTS The uHG was significantly faster in spot sign patients (p < 0.001), as well as in patients who experienced HG (p = 0.021), END (p < 0.001), 3-month mortality (p < 0.001), and poor long-term outcome (p < 0.001). The uHG improved the accuracy of baseline ICH volume in the prediction of END (sensitivity 93.1% vs 82.8%, specificity 85.3% vs 82.4%) and 3-month mortality (sensitivity 77.5% vs 70%, specificity 87.9% vs 84.6%). A uHG >10.2 mL/hour emerged as the most powerful predictor of HG (odds ratio [OR] 3.55, 95% confidence interval [CI] 1.39-9.07, p = 0.008), END (OR 70.22, 95% CI 14.63-337.03, p < 0.001), 3-month mortality (OR 16.96, 95% CI 5.32-54.03, p < 0.001), and poor long-term outcome (OR 6.19, 95% CI 1.32-28.98, p = 0.021). CONCLUSIONS The uHG represents a powerful and easy-to-use tool for improving the prediction of HG and outcome in patients with acute ICH.