Background: The prevalence of Gastroesophageal Reflux Disease (GERD) in Turkey is reported as 11.6%. Studies of pulmonary function in asthmatics have demonstrated a correlation between lung resistance and the occurrence of spontaneous gastroesophageal reflux. Few studies have included measures of lung diffusing capacity for carbon monoxide. The aim of this study is to assess whether asthma patients had worse lung function and gas diffusion according to diversity of GERD symptoms they concurrently experienced. The secondary aim of the study is to determine the frequency and different faces of GERD in our asthma patients compared to healthy controls.