OBJECTIVE: Differential diagnosis of idiopathic pulmonary fibrosis (IPF) is important among fibrotic interstitial lung diseases (ILD). Thisstudy aimed to evaluate the rate of IPF in patients with fibrotic ILD and to determine the clinical-laboratory features of patients with andwithout IPF that would provide the differential diagnosis of IPF. MATERIAL AND METHODS: The study included the patients with the usual interstitial pneumonia (UIP) pattern or possible UIP patternon thorax high-resolution computed tomography, and/or UIP pattern, probable UIP or possible UIP pattern at lung biopsy according tothe 2011 ATS/ERS/JRS/ALAT guidelines. Demographics and clinical and radiological data of the patients were recorded. All data recordedby researchers was evaluated by radiology and the clinical decision board.RESULTS: A total of 336 patients (253 men, 83 women, age 65.8±9.0 years) were evaluated. Of the patients with sufficient data for diag nosis (n=300), the diagnosis was IPF in 121 (40.3%), unclassified idiopathic interstitial pneumonia in 50 (16.7%), combined pulmonaryfibrosis and emphysema (CPFE) in 40 (13.3%), and lung involvement of connective tissue disease (CTD) in 16 (5.3%). When 29 patientswith definite IPF features were added to the patients with CPFE, the total number of IPF patients reached 150 (50%). Rate of male sex(p<0.001), smoking history (p<0.001), and the presence of clubbing (p=0.001) were significantly high in patients with IPF. None of thewomen <50 years and none of the men <50 years of age without a smoking history were diagnosed with IPF. Presence of at least 1 of thesymptoms suggestive of CTD, erythrocyte sedimentation rate (ESR), and antinuclear antibody (FANA) positivity rates were significantlyhigher in the non-IPF group (p<0.001, p=0.029, p=0.009, respectively).