AIM: This research aimed to evaluate the effect of gastroesophageal reflux disease (GERD) on pulmonary volumes, airflows, and airway resistance in the patients without respiratory symptoms and compare them with the healthy subjects. BACKGROUND: GERD is the return of gastric content into the esophagus and beyond. GERD may play an essential role in the extraesophageal diseases, including chest pain, asthma, laryngitis, chronic cough, and sinusitis. The relation between GERD and airway involvement in asthma and also bronchoconstrictor effects of GERD are well recognized, but its impact on lung parameters in the patients with GERD without respiratory symptoms is unclear. METHODS: In a case-control study, 78 GERD patients without pulmonary symptoms and 93 healthy subjects as control group were enrolled. The impulse oscillometry examined airway resistance. The body plethysmograph measured the pulmonary volumes and airflows. RESULTS: The mean age of GERD patients and the healthy subjects were 37.30±9.76 and 34.74±11.10, respectively. A total of 53.8% of patients and 67.7% of healthy subjects were male. The lung volumes measured by the body plethysmography were normal in both patients and healthy subjects. However, there was a significant difference between the groups in forced expiratory volume in 1 second (FEV1)/forced vital capacity (FVC) (P=0.01) and maximal mid expiratory flow (MMEF) (P=0.008). Airway resistance at R5Hz was significantly higher in the case group than the control group (P=0.001). CONCLUSION: The results of the current study demonstrated that GERD patients have small airway disease even in the absence of respiratory symptoms.