We examined the patency of the ostium of the frontal sinus after endoscopic endonasal surgery for chronic sinusitis. This study involved one hundred and seventy-two nasal sides of ninety cases who underwent surgery in this unit in the preceding three-year period by the designated three surgeons. All patients were followed up for more than one year after surgery. We obtained a high postoperative patent rate of 90.1%. However, communication between the frontal and ethmoidal sinus could not be confirmed in 9.9% of the cases due to the presence of polyp or adhesion in the middle meatus. Cases with preoperative severe lesion of the frontal sinus showed significantly lower rates of patent than cases with preoperative mild and/or no lesion. In cases where the opening of the ostium could not be sufficiently widened during surgery because the size around the ostium was already small, lower patent rates resulted. However, even in such cases, scraping or curetting of the surrounding bone should be avoided, because it may cause postoperative narrowing of the ostium due to new bone formation. Cases with unsatisfactory results caused by pathological changes in the middle meatus showed significantly lower patent rates than cases with satisfactory postoperative results. It is recommended that accurate cleaning of the ethmoid sinus and adequate postoperative treatment be considered important. Endoscopic endonasal opening of the ostium of the frontal sinus is shown to be a safe and reliable procedure, which can be performed with clear visualization.