The colloidal osmotic and hydrostatic pressure of chronic subdural hematomas were measured with a manometer in 35 patients. Manometric measurement showed an increase in the hematoma fluid corresponding to 5% of the total hematoma volume. The maximum colloidal osmotic pressure of the hematoma fluid, as measured with a transducer, was 23 ± 12 mmHg (mean ± standard deviation). Hemorrhage occurs continuously or intermittently within a subdural hematoma, accompanied by red cell hemolysis and hemoglobin degradation. The colloidal osmotic pressure within a subdural hematoma is higher than that in plasma or cerebrospinal fluid, so that the hematoma is able to absorb water through semipermeable membranes or capillaries. However, since the absorption force can transport only 5% of the hematoma volume, the colloidal osmotic pressure is a contributory process rather than a primary etiologic factor.