Red cell survival studies using an infusion of chromium-51-labeled erythrocytes were performed to quantitatively estimate hemorrhage in the chronic subdural hematoma cavity of 50 patients. The amount of hemorrhage was determined during craniotomy. Between 6 and 24 hours after infusion of the labeled red cells, hemorrhage accounted for a mean of 6.7% of the hematoma content, indicating continuous or intermittent hemorrhage into the cavity. The clinical state of the patients and the density of the chronic subdural hematoma on computerized tomography scans were related to the amount of hemorrhage. Chronic subdural hematomas with a greater amount of hemorrhage frequently consisted of clots rather than fluid.