The favorable effect of burr hole irrigation and closed drainage system has been reported in the treatment of chronic subdural hematoma (CSH). After analyzing the relationship of the direction of the drainage tube and recurrence, we have suggested that the residual air after surgery night be important as one of the factors causing recurrence. The present study included 128 chronic subdural hematomas in 102 patients who had been treated in our institute between January, 1996 and October, 2000. We attempted to place the tip of the external drain at the occipital side within the hematoma cavity between January, 1996 and December, 1998 (occipital drainage group), while we tried to insert the tip of the external drain at the frontal side using an endoscope between January, 1999 and October, 2000 (frontal drainage group). We focused on the residual air on CT, the direction of the drainage, recurrence of the hematoma and the size of the hematoma. Some statistical analyses were conducted using these factors. The recurrence rate was slightly lower in the frontal drainage group than that in occipital drainage group. There was no difference in the residual rate of air immediately after the surgery on CT and the recurrence rate between the 2 groups. However, the interval preceding the recurrence was statistically significantly longer in the frontal drainage group. A higher rate of recurrence was noted for cases in which the volume of hematoma was more than 70 ml. The residual rate of air 1 week after surgery was significantly higher in the recurrent group than in non-recurrent group. In this study, it is demonstrated that a long-standing residual of air after surgery is one of the causes of the recurrence of CSH. We must contrive to reduce the residual air after surgery for the treatment of CSH.