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 (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. CI= : confidence interval; CTA= : CT angiography; END= : early neurologic deterioration; GCS= : Glasgow Coma Scale; HG= : hematoma growth; ICH= : intracerebral hemorrhage; IQR= : interquartile range; mRS= : modified Rankin Scale; NIHSS= : NIH Stroke Scale; OIT= : onset-to-imaging time; OR= : odds ratio; ROC= : receiver operating characteristic; uHG= : ultraearly hematoma growth