The oncotic pressure of fluid from subdural hematomas and subdural hygromas was compared to that of simultaneously drawn venous blood in 20 patients. There was no significant difference in the oncotic pressure of fluid from subdural hematomas and venous blood; however, the oncotic pressure of fluid from subdural hygromas was significantly less than that of blood. This finding fails to support the Zollinger and Gross modification of Gardner's that chronic subdural hematomas grow and produce symptoms after a latent interval because they attract fluid from the blood via dural vessels.