Chronic subdural hematoma (CSDH) is generally treated by twist drill, and one and two burr-hole craniostomy. We proposed new classification of the intraoperative condition of CSDH, and present a safer technique for aspiration of CSDH in one burr-hole surgery. The intraoperative condition of CSDH was classified according to the connections between the hematoma cavity and the extracranial space as follows. The "closed condition" represents only a single route consisting of a tube inserted intraoperatively connecting the extracranial space to the hematoma cavity. The "open condition" includes another route or space, which can freely pass air, saline, or old hematoma fluid, in addition to the tube inserted intraoperatively. Twist drill craniostomy and two burr-hole craniostomy clearly involve the intraoperative closed and open conditions, respectively. One burr-hole craniostomy may involve either condition due to the operative procedure. Aspiration and irrigation of the hematoma is basically free and safe in the open condition, but risky in the closed condition. All of the hematoma can be aspirated through one burr hole under certain open conditions with temporary replacement of the hematoma cavity with air followed by replacement of air with saline. Twenty-seven patients with symptomatic CSDH underwent one burr-hole craniostomy by the above mentioned aspiration technique. There were no special complications. The recurrence rate was average. The substitution of saline after complete aspiration of hematoma carries little risk only under the "open condition," shortens the operation time, and achieves good irrigation of the hematoma.