Objective : This study aims to compare the potential sinus distribution between high-volume nasal irrigation and nasal spray in chronic rhinosinusitis patients who have not undergone sinus surgery. Design and Setting : A randomized clinical study was conducted at the Otolaryngology-Head & Neck Surgery Department, Ramathibodi Hospital, Faculty of Medicine, Mahidol University. Participants : 40 patients undergoing endoscopic sinus surgery for chronic rhinosinusitis. Thirty-eight patients met the inclusion criteria and were randomly assigned to receive nasal irrigation or nasal spray mixed with fluorescein sodium preoperatively. Main outcome measures : The primary outcome was the mean difference in the staining score of fluorescein in all sinuses between the two groups. Results : The total staining score of fluorescein in all sinuses via nasal irrigation was statistically more significant than the score via nasal spray, with a mean difference score of 2.90, 95%CI: 1.22-4.58, p-value 0.001. The most significantly affected sinuses were specific to the maxillary and anterior ethmoid sinuses, whereas the frontal and sphenoid sinuses had the slightest solution distribution from both techniques. Conclusion : Nasal irrigation is a potential route to deliver drugs into the sinus in unoperated CRS patients. However, it is not considered a superior method to nasal spray at the most challenging anatomical areas, i.e., the frontal and sphenoid sinuses. Keywords : Corticosteroid use, Endoscopic sinus surgery, Irrigations, Topical therapy for chronic rhinosinusitis, Medical therapy for chronic rhinosinusitis, Nasal polyp, Nasal spray, Chronic rhinosinusitis