Background and purpose Patients with acute intracerebral hemorrhage (ICH) pretreated with antithrombotic drugs may have increased early hematoma growth, which would increase mortality risk. The effect of antiplatelet (AP) and vitamin K antagonist (VKA) pretreatment on ultra‐early hematoma growth (uHG) and its relationship with mortality in patients with acute supratentorial ICH was analyzed. Methods This is an observational retrospective study of a prospective register of 197 ICH patients with first computed tomography (CT) scan taken <6 h from ICH symptom onset. ICH volume was calculated by the ABC/2 formula and uHG by the baseline ICH volume/onset‐to‐CT time (ml/h) formula. The uHG analysis took into account the patient's pretreatment (none, AP or VKA) and the relationship between uHG and very‐early (first 24 h) and 3‐month mortality. Results In the pretreatment group, 50 (25.4%) patients were treated with AP and 37 (18.8%) with VKA. The median (interquartile range 25–75) uHG was 19.7 ml/h (2.9–44.8) for AP pretreated patients, 16.2 ml/h (5.1–42.5) for VKA pretreated patients and 8.4 ml/h (2.4–21.8) for non‐pretreated patients, P = 0.019. The uHG was higher in patients with very‐early 42.1 ml/h (20.1–79.6) and total 3‐month mortality 28.0 ml/h (15.8–52.5) compared with survivors 3.9 ml/h (1.5–10.4), P < 0.0001. Adjusted by ICH severity and previous functional status, uHG was an independent factor related to very‐early (P = 0.028) and total 3‐month mortality (P = 0.014). Conclusions Patients pretreated with antithrombotics have much higher uHG, which would explain the increased mortality in these patients compared to untreated patients.