Twenty-two patients with chronic subdural hematoma were treated with simple closed drainage between July 1983 and June 1984. Fibrinogen and fibrinogen degradation products(FDP's) were measured in the hematoma fluid. The relationship of FDP's to the time interval from trauma to operation, to the volume of hematoma fluid, to the duration of postoperative drainage, and to the expansion rate of the brain were investigated. The results were as follows : 1) Significantly high values of FDP's and relative low values of fibrinogen were observed on the hematoma fluid. 2) Values of FDP's elicited had no relationship to the time interval from trauma to operation, to density on CT scan, nor to the expansion rate of the brain. 3) Values of FDP's showed a close relationship to the volume of the hematoma and to the duration of postoperative drainage.