Fourty six patient with chronic subdural hematoma(CSDH) were managed by simple closed drainage between January 1987 and January 1990. The osmolarity and its factors of fluid from CSDH were compared to that of simultaneously drawn venous blood in patients. The results were as follows ; 1) The values of K+ and albumin of the hematoma fluid were significantly higher than that of hematoma fluid(P<0.05). 2) Osmolarities of the hematoma fluid were higher than that of venous blood(P<0.01). 3) Osmolarity, K+ and albumin showed linear relationship between hematoma fluid and venous blood(P<0.01).