Purpose: To evaluate the dynamic patency of the frontonasal duct(FND) on PNS CT scan after administration of an iso-osmolar contrast agent (lsovist) into the nasal cavity. Materials and Methods: Coronal PNS CT scans from ten normal and 30 patients with symptoms of chronic sinusitis were obtained after administration of lsovist into the nasal cavity, followed by Valsalva maneuver for the even distribution of the agent into the FND and sinuses. A total of 80 FNDs were evaluated for dynamic patency by examining the presence of the contrast agent in ducts. Results: The contrast agent was visualized in 34/37 FNDs(91.9%) without ipsilateral frontal sinusitis and was visualized in 18/43 FNDs(41.9%) in cases of ipsilateral frontal sinusitis. In addition, the contrast agent within the ipsilateral frontal sinus was visualized in 8/18 FNDs(44.4%). The major causes of ductal obstruction weremucoperiosteal thickening and polypoid lesions secondary to inflammation. Conclusion: PNS CT scan together with the administration of lsovist, which caused minimal irritation, was useful for the evaluation of the relationship between the dynamic patency of the FND and frontal sinusitis, In addition, this study can provide the rhinologist with accurate anatomical and pathological information concerning the FND before a choice is made between endoscopic sinus surgery or medical treatment.