OBJECTIVE: To identify the prognostic factors that may affect the outcome of endoscopic sinus surgery (ESS) in chronic rhinosinusitis (CRS). METHOD: Study design: Prospective study 68 patients having CRS were studied (over a priod of 2 years) including history taking, endoscopic examination, CT and measurement of nasal nitric oxide (nNO),nasal carbon monoxide (nCO) and total nasal airway resistance. All patients underwent ESS. The specimens were studied histopathologically (for eosinophilia)and bacteriologically (for intracellular S. aureus). RESULTS: Intracellular S. aureus, old age, tissue eosinophilia, and Widal syndrome were found to be associated with a less favorable long-term outcome. In contrast, anatomic variants, allergy, bronchial asthma, and smoking do not seem to affect the prognosis of ESS in patients with CRS. Post-operatively, there was a significant increase in nNO and decrease in nCO production. Patients with polyposis showed less nNO level and olfactory threshold, but higher CT and endoscopic scores. The CT score correlated positively with the endoscopic score, the surgery score, and the degree of tissue eosinophilia. CONCLUSION: 1) The prognostic factors that may affect the outcome of ESS in CRS have been identified. 2) The postoperative increase in nNO and decrease in nCO levels raises the question of whether they can be used to monitor the treatment of CRS like their use in bronchial asthma or not. 3) The olfactory deficit in patients with polyposis is most likely due to mucosal inflammation rather than airway obstruction.