The goal of this study is to assess the effect of functional endoscopic sinus surgery (FESS) on spirometry results in patients with chronic rhinosinusitis (CRS) and chronic obstructive lung disease. In Otorhinolaryngology and chest departments in Beni-Suef University hospital from August 2016 to February 2019, 60 patients were included in this study: 47 were asthmatic and 13 were chronic obstructive pulmonary disease (COPD) and all had chronic rhinosinusitis with nasal polypi, preoperative spirometry and computerized tomography (CT) were done. Lund-MacKay CT score system was used. After FESS by comparing results of preoperative spirometry in Forced Expiratory Volume at 1^st second percentage (FEV1%) and Forced Expiratory Volume at 1^st second/ Forced Vital Capacity percentage (FEV1/FVC%) and Lund-MacKay CT score, there was a statistical significance in improvement of FEV1% and FEV1/FVC% on 1^st and 3^rd months postoperative in both asthmatic and COPD patients and there was a statistical significance in decrease of Lund-MacKay CT score on 3^rd month postoperatively.