Recurrence of chronic subdural hematoma (CSH) is a significant problem in neurosurgical practice. Various risk factors associated with patient's characteristics and the pathogenesis of CSH have been investigated in many studies, but controversial findings are not uncommon. Therefore we made a retrospectively investigation focusing mainly on the factors associated with surgical techniques. in order to find out the factors that may affect the recurrence rate of CSH. The medical records of 97 consecutive patients with 121 CSHs, who were treated with burr-hole craniostomy and continuous drainage, were retrospectively reviewed. The relationships between the recurrence rate and some factors associated with surgery, such as location of burr-hole, thickness of residual hematoma, location of drainage catheter, intracapsular air postsurgery, duration of drainage were investigated. The average recurrence rate was 6.6% in this whole series. For patients with less than three days of drainage, the recurrence rate was 16.3%, whereas for those with 3 and more days of drainage, the recurrence rate was only 1.3%. The duration of drainage significantly related to recurrence rate. In addition, a higher recurrence rate seemed to be associated with more intracapsular air postsurgery, but it did not reach statistically significant level in this study. Prolonged duration of drainage did not increase the frequency of infection in this series. We found in this study that the duration of drainage play an important role in the treatment of CSH and 3 full days of drainage seems to be necessary, especially for patients of 60 years and over. We presumed that it takes at least 3 days of drainage for outer membrane of CSH to get the restoration of a normal balance between coagulation and fibrinolysis after surgery, which is necessary for termination of the vicious cycle and resolution of the hematoma.