Rationale and background: Rhinosinusitis, an inflammatory disease involving the nasal cavity and paranasal sinuses, affects millions of individuals. The development of rigid nasal telescopes has revolutionized the diagnosis and treatment of this disease.Functional Endoscopic sinus Surgery (FESS), nowadays, becomes the most accepted surgery for chronic rhinosinusitis. The results of FESS are difficult to interpret because of large number of variable factors.Objectives: This thesis was designed to recognize the out-come of ESS in cases of chronic rhinosinusitis (CRS) and to know the incidence and causes of recurrence.Methods: Prospective review of 40 patients with CRS with failed medical treatment requiring FESS presenting in Kasr El-Aini hospital. All patients are subjected to history taking, clinical examination including endoscopy, subjective evaluation of symptoms and CT-san.All patients were candidate to FESS by Messerklinger’s technique with follow up for six months where another subjective evaluation of symptoms and objective assessment including endoscopy , mucociliary clearance test and CT- Scan were done .Results : six patients (15%) were identified of recurrence of CRS and needed revision surgery . The most common cause of failure was residual air cells and adhesions in the ethmoid area (33%) and missed frontal Pathology (33%). Study also , showed 29 patients became symptom free (72,5%), 10 patients became symptom free e. additional therapy (medical or revision surgery ) (25%), and only one patients was improved with maintenance (medical treatment) (2.5%). Conclusion : Functional Endoscopic Sinus Surgery (FESS) still has an incidence of failure , so the patient's symptoms must be put in mind before doing FESS and also put in mind the most common causes of surgical failure are : adhesions , residual air calls , missed frontal pathology and maxillary ostium stenosis to be avoided during surgery and follow – up .