Obstructive sleep apnea (OSA) and chronic obstructive pulmonary disease (COPD) are two of the most common chronic respiratory disorders. Co-existence of both conditions, referred as overlap syndrome (OLS), is associated with substantially high rates of mortality and morbidity. The present study aimed to evaluate the effect of inhaled corticosteroids (ICS) on apnea-hypopnea index (AHI), an indicator for diagnosis and identifying the severity of OSA, in overlap syndrome.We conducted a clinical trial on 60 patients diagnosed with overlap syndrome by employing overnight polysomnography before and after receiving ICS. Tstudent test and Mann-Whitney test were applied to analyze the gathered data including age, AHI, nocturnal oxygen desaturation index and SaO2, (saturated arterial oxygen), daytime (pressure of arterial carbon dioxide) PaCO, level, forced expiratory volume in one second (FEV1), body mass index (BMI), and waist and neck circumferences.By 3-month ICS administration, this study demonstrated significant reduction of mean AHI and nocturnal oxygen desaturation index along with remarkable improvement of FEV1, diurnal PaCO2, level and nocturnal SaO2, (P< 0.05). Meanwhile, BMI and waist and neck circumferences measurement showed no noticeable changes.As we have not found any literature demonstrating, this is the first study which has evaluated the effect of ICS on AHI in overlap syndrome. Because of a remarkable improvement in obstructive sleep apneas, this study suggests that ICS might be beneficial in treatment of overlap syndrome.