The pathophysiology of chronic subdural haematomas (CSH) is still unclear. In the light of recent ultrastructural examination, exudation from the macrocapillaries in the outer membrane of CSH may play an important role in the enlargement of CSH. In this study, exudation from the macrocapillaries was assessed by the measurement of phenytoin, a protein-bound antiepileptic agent used in cases of CSH. In 22 patients, 1 h after the administration of 250 mg of phenytoin intravenously, blood and subdural haematoma samples were taken and phenytoin levels were measured. The ratio of subdural haematoma level to the blood phenytoin level was determined and defined as the phenytoin penetration ratio (PPR). The correlation between the phenytoin penetration ratio and clinical neurological grades (Markwalder and Glasgow Coma Scale), age of the patients and the CT appearance of CSH were investigated. The mean phenytoin penetration ratio was 19.5%. As the neurological grades of patients increased, average PPR also increased. The average PPR values were 17.64 and 20.84% in the patients younger than 60 years (nine patients) and older patients (13 patients), respectively. Mean PPRs in the groups according to the CT appearance were as follows: low density 11.21% (seven patients), isodensity in 15.88% (10 patients), high density in 38.5% (five patients). A subdural reaccumulation was detected in nine patients with a mean PPR of 27.72%, while mean PPR was 14.56% in the others. Exudation from macrocapillaries in the outer membrane of chronic subdural haematomas probably plays an important role in the enlargement of chronic subdural haematoma, and measuring phenytoin levels in the chronic subdural haematoma is a simple method for the quantitative estimation of the exudation in CSH.