Pulmonary fibrosis is the formation or development of excess fibrous connective tissue (fibrosis) in the lungs. It is also described as scarring of the lung. Symptoms of pulmonary fibrosis are shortness of breath, chronic dry hacking coughing, fatigue and witness, chest discomfort, loss of appetite and weight loss.

Pulmonary fibrosis may be a secondary effect of other diseases. Most of these are classified as interstitial lung diseases. Examples include autoimmune disorders, viral infections or other microscopic injuries to the lung. However, pulmonary fibrosis can also appear without any known cause. In this case, it is termed idiopathic. Most idiopathic cases are diagnosed as idiopathic pulmonary fibrosis. This is a diagnosis of exclusion of a characteristic set of histologic/pathologic features known as usual interstitial pneumonia.