Idiopathic pulmonary fibrosis (IPF), or (formerly) fibrosing alveolitis, is a rare, progressive illness of the respiratory system, characterized by the thickening and stiffening of lung tissue, associated with the formation of scar tissue. It is a type of chronic scarring lung disease characterized by a progressive and irreversible decline in lung function. The tissue in the lungs becomes thick and stiff, which affects the tissue that surrounds the air sacs in the lungs. Symptoms typically include gradual onset of shortness of breath and a dry cough. Other changes may include feeling tired, and abnormally large and dome shaped finger and toenails (nail clubbing). Complications may include pulmonary hypertension, heart failure, pneumonia or pulmonary embolism.

The cause is unknown, hence the term idiopathic. Risk factors include cigarette smoking, acid reflux disease (GERD), certain viral infections, and genetic predisposition. The underlying mechanism involves scarring of the lungs. Diagnosis requires ruling out other potential causes. It may be supported by a HRCT scan or lung biopsy which show usual interstitial pneumonia (UIP). It is a type of interstitial lung disease (ILD).

People often benefit from pulmonary rehabilitation and supplemental oxygen. Certain medications like pirfenidone (Esbriet) or nintedanib (Ofev) may slow the progression of the disease. Lung transplantation may also be an option.

About 5 million people are affected globally. The disease newly occurs in about 12 per 100,000 people per year. Those in their 60s and 70s are most commonly affected. Males are affected more often than females. Average life expectancy following diagnosis is about four years. Updated international guidelines were published in 2022, which some simplification in diagnosis and the removal of antacids as a possible adjunct therapy.