Introduction: IPF is defined as a specific form of chronic fibrosing interstitial pneumonia limited to the lung and associated with histological appearance of usual interstitial pneumonia(UIP) on surgical(thoracoscopic or open) lung biopsy. The etiology is unknown. Pulmonary function tests are a very important clinical tool for diagnosis, case management, evaluation, and response to therapy of various cardiopulmonary diseases, as well as being a part of the health screening process. The introduction of high-resolution computed tomography (HRCT) has revolutionized the radiologic approach to patients with suspected diffuse infiltrative lung disease (DILD). The requirements for performing HRCT include a current technology scanner, thin slices (1–2 mm), and reconstruction using a high spatial frequency algorithm.Subjects & Methods: The thesis included 30 patients diagnosed to have Idiopathic Pulmonary Fibrosis. Six patients were excluded due to failure to do Carbon Monoxide diffusion and two were normal. All Patients were subjected to Spirometry, Carbon monoxide diffusing capacity (DLCO) using single breath technique and High Resolution CT chest. Extension of involvement was assessed at 5 levels (the origin of great vessels, the mid arch of the aorta, the main carina, the pulmonary venous confluence, and 1 cm above the right dome of diaphragm). Correlation between HRCT abd PFT were done among all variables.Disscussion: There was a significant negative correlation between KCO % and HRCT chest affection (r=-0.485, P=0.016), there was a significant positive correlation between FEV1/FVC and HRCT chest affection among all patients. Among patients with restrictive defects, the mild group showed Strong significant inverse correlation was found between KCO and CT% affection (r=-0.0986, p=0.000), moderate group showed significant strong positive correlation was found between CT% affection and FEV1/FVC (r=0.664, p=0.026), and strong significant inverse correlation between CT% affection and Kco (r=-0.945, p=0.000), and severe group showed strong inverse correlation between CT% affection and FVC (r=-0.9, p=0.01), strong significant inverse correlation between CT% affection and FEV1 (r=-0.9, p=0.01), as well as for strong significant inverse correlation between CT% affection and FEF 25-75% (r=-0.9, p=0.01), and strong significant inverse correlation between CT% affection and Kco (r=-0.9, p=0.01). This is the full blown picture and findings in advanced Idiopathic pulmonary fibrosis. Among Diffusion defect, strong significant inverse correlation between FEF 25-75% and CT% affection (r=-0.975, p=0.005) and strong significant inverse correlation between KCO% and CT% affection (r=-0.975, p=0.005)in mild group, for moderate group there was Strongly significant positive correlation between FEV1/FVC and CT% affection (r=0.611, p= 0.026), and strong significant inverse correlation between Kco and CT% affection (r=-0.666, p= 0.013) and severe group showed strong significant inverse correlation between FVC% and CT% affection (r=-0.97, p=0.03), also strong significant inverse correlation between FEV1% and CT% affection (r=-0.97, p=0.03), strong significant inverse correlation between FEF 25-75% and CT% affection (r=-0.92, p=0.04) and finally for the correlation between Kco and CT% affection there was strong significant inverse correlation (r=-0.92, p=0.04). Conclusion: This study concludes that CT% affection based on the radiologist experience correlated with Spirometry and diffusion test were sensitive methods in assessing the severity of idiopathic pulmonary fibrosis.