Bronchiolitis obliterans with organizing pneumonia (BOOP) is an inflammation of the small airways (bronchioles) and surrounding tissue in the lung. This inflammation can affect a small area of the lung or the entire lung. It's not associated with infection, smoking or lung cancer.

The term obliterans refers to the fact that the inflammation of the bronchioles partially obliterates the small airways. The "pneumonia" isn't infectious or from an organism but is due to inflammation of lung tissue around the inflamed bronchioles. A more accurate term to describe this would be pneumonitis but pneumonia is the term that has stuck.

In many cases, the cause of BOOP can't be determined. But known causes include:

• Certain medications
• Radiation therapy, especially for breast cancer
• Organ and tissue transplantation, especially bone marrow transplants
• Certain connective tissue disorders, such as lupus

In some people, BOOP may cause no signs or symptoms. It may be found incidentally on a chest X-ray done for some other reason. In others, it may cause shortness of breath and fever. Signs and symptoms usually develop slowly over several weeks.

This condition is difficult to diagnose. A doctor may make a diagnosis of BOOP based on:

• Personal medical history
• High-resolution computerized tomography (CT) scan of the lungs
• Removal of lung tissue (biopsy) for examination under a microscope

Treatment depends on the underlying cause but may include prednisone.