OBJECTIVE: Patients receiving anticoagulation therapy develop intracerebral hematomas, which are difficult to evacuate during endoscopic surgery. Insufficient thrombin activation results in the gradual conversion of fibrinogen to fibrin, forming dense fibrin bands (FBs) that harden the hematoma. We aimed to investigate the factors contributing to hematoma hardness. METHODS: Eleven men and 11 women (age range: 41-84 years) with hypertensive intracerebral hematomas underwent surgery between 2016 and 2018. Hematoma hardness was measured intraoperatively as the maximum breaking point under compression (Newton [N]) using a bar-type tester. Hematoma samples were graded based on histological FB density. The relationships between hardness, FB grade, and hematological parameters were assessed in patients with and without anticoagulation therapy. RESULTS: Hematomas were categorized as non-hard (0-3.0 N, n = 16) or hard (3.0-7.0 N, n = 6). Hardness was strongly correlated with FB grade (r = 0.76) and prothrombin time-international normalized ratio (PT-INR) (r = 0.75) and moderately correlated with activated partial thromboplastin time (APTT) (r = 0.65). Three patients with hematoma receiving warfarin and one receiving factor Xa inhibitor demonstrated significantly higher FB grade (p = 0.008) and increased hematoma hardness (6.0 ± 1.4 N) compared with those without anticoagulation treatment (1.3 ± 1.2 N, p = 0.00001). Receiver operating characteristic curve analysis indicated that a PT-INR of 1.81 and an APTT of 32.3 s may predict hard hematomas. CONCLUSION: Tough hematomas were histologically characterized by high FB density. Preoperative PT-INR and APTT may predict the presence of tough hematomas.