Surgery for chronic subdural hematoma (CSDH) is performed to relieve brain displacement and high intracranial pressure (ICP). However, the intraoperative impression is often that the pressure inside the CSDH is low, despite marked clinical symptoms. We wanted to quantify the CSDH pressure and relate this to radiological and clinical characteristics. This prospective, population-based study of unilateral CSDHs was conducted over a 3-year period. CSDHs that were secondary to other conditions, re-operations, or CSDHs requiring other procedures than burr hole craniostomy under local anesthesia were excluded. Subdural pressure registration was performed via a simple manometric technique, and full compliance with a standardized protocol was mandatory. Sixty patients were included (mean age 76.2 years; for men, 77.4, and for women, 72.9). The mean pressure in the CSDHs was 15.2 cm H(2)O (range, 0-40) with no gender difference. Men had significantly larger volumes (mean 158.1 vs. 103.2 cm(3)) and midline shifts (mean 1.04 vs. 0.68 cm) than did women. Large hematomas with large midline shifts had higher pressures and more often required repeat surgery. With a patient's increasing age, the volumes and midline shifts seemed to become larger, whereas the pressures became lower. We did not find an association between repeat surgery and pressure or age. Our results are generally in line with those of previous studies reporting quantitative pressure registrations. However, there are important disparities regarding methodology, not least when comparing with various subjective scales that are widely used in clinical practice. A mean subdural pressure of 15.2 cm H(2)O is probably within the range of a normal ICP.