Rationale The association between interstitial lung disease (ILD) and obstructive sleep apnea (OSA) is commonly encountered in clinical practice. ILD diagnosis can be easily missed in those patients. Objective To investigate for predictors of ILD in patients with OSA. Methods This is a prospective observational study. 309 patients presented to polysomnography unit in Assiut University Hospital, diagnosed with OSA [Respiratory Disturbance Index (RDI) more than 5] were reviewed. High resolution computed tomography (HRCT) chest was done for all included patients to screen for ILD. Spirometry was done to evaluate the severity of the restriction. Echocardiography was performed by a cardiologist to screen for pulmonary hypertension. Results In all 228 (73.8%) patients had normal HRCT. Eighty-one (26.2%) cases were found to have features of ILD in HRCT chest. Patients with subclinical ILD 73 (90.1%) cases had significantly higher RDI when compared with known cases of ILD 8 (9.9%) cases. Their level of PO2 and forced vital capacity (FVC) were significantly reduced (P=0.003, and