Objective : Arachnoid cysts (ACs) can be cured by making the definite and wide communication between the cyst and arachnoid space using endoscopy, but often it is impossible only through the usual working-channel (intra-channel) procedures. We discuss and propose a more valuable endoscopic technique with the presentation of our series of cases. Methods : We treated 9 patients with cortical AC in various locations with extra-channel endoscopic techniques. The patients ranged in age from 3 years to 60 years (mean age, 37.2 yrs). The follow-up period ranged from 12 to 26 months (mean follow-up duration, 17.2 months). All patients had large AC compressing the adjacent brain with clinical symptoms or signs. The authors performed extensive fenestration via single burr hole with the aid of endoscope. Being bypassed the rigid endoscope, through the space between the shaft of endoscope and guiding cannula (extra-channel method), fenestration procedures were done in the dry fields. Results : Eight (88.9%) patients had been treated successfully with endoscope. One patient required shunt procedure. Among the eight patients who were treated with endoscopic procedure, 6 patients (66.7%) showed cyst reduction, and two (22.2%) showed disappearance of cyst. Conclusion : We suggest that extra-channel method will be simple and easy to perform using more valuable instruments with wider working area, and may promise better results compared to the conventional intra-channel endoscopic procedures.