Background: Chronic rhinosinusitis (CRS) significantly affects the quality of life of patient. Chronic rhinosinusitis (CRS) is common chronic condition and is treated with antibiotics, nasal irrigation and steroids. Patients who do not respond to medical therapy are subjected to functional endoscopic sinus surgery. Objective: To assess the clinical improvement after functional endoscopic sinus surgery among the patients of chronic rhinosinusitis (CRS) who failed medical management. Methods: A Prospective longitudinal study conducted in ENT department of Mission General Hospital, Nilphamary, Bangladesh. The study period was from March 2020 to May 2020. Patient with CRS who failed with medical management were subjected functional endoscopic sinus surgery (FESS). Pre –operative symptoms score and endoscopic score were assessed and compared with that of post- operative scores at the end of 6 weeks and 12 weeks. Results: Fifty-One patients who completed three months of follow up were included in the study. The mean preoperative VAS symptoms score was 5.7 and was 2.23 after FESS and difference was significant (P <0.01). There was significant improvement of the endoscopic score (ES) after FESS, the mean ES preoperatively was 4.038 and it improved to1.31 at 6 weeks and 0.75 in the 12 postoperative weeks. Post-operative adhesion was the most common complication and occurred in 15% of patients. Conclusions: FESS is a safe procedure. Patients with CRS who don’t respond to medical therapy should undergo this safe surgical procedure FESS with good outcome.