Frontal sinusotomy was performed on 110 patients undergoing routine endoscopic endonasal ethmoidectomy and the minimum diameter of the frontal sinus neo-ostium was determined intraoperatively. A total of 82 patients could be subjected to follow-up and redetermination of the neo-ostium diameter 13 months later. A postoperative CT was scheduled in 62 cases. The average minimum diameter of the frontal sinus neo-ostium, measured intraoperatively, was 5.6 mm (0-11 mm). After completion of wound healing, 81% of the frontal sinuses could be explored by probing or even inspected by rigid endoscopy. The average minimum diameter of the neo-ostia determined postoperatively was 3.5 mm (0-11 mm). Patients exhibiting aspirin sensitivity or diffuse nasal polyposis showed a more pronounced scarred constriction of the frontal sinus access compared to other cases. Neo-ostia exceeding 5 mm intraoperatively were preserved with a considerably higher percentage than those with diameters of less than 5 mm. Radiologically, the fenestrated frontal sinuses frequently showed continued or even increasing mucosal congestion. No conclusive relationship was found to exist between such post-operative clouding and frontal sinus accessibility (endoscopy and/or probing) or patient complaints. The investigations confirm the safety and reliability of frontal sinusotomy in surgical management of chronic paranasal sinusitis. The mucosa of the frontal sinus often reacts to surgery in the form of persistent or even newly developing mucosal swelling to which a specific pathophysiological significance cannot always be attributed.