Background: Interstitial lung disease affects the tissues that aid the transfer of oxygen and carbon dioxide between the air and the bloodstream. The disease can cause fibrosis, a thickening and scarring of lung tissue. Fibrosis often continues getting worse, and most people with this disease die in 3 to 5 years.

Objective: To test a study drug (hymecromone) in people with interstitial lung disease or lung fibrosis.

Eligibility: People aged 18 years and older with interstitial lung disease or lung fibrosis.

Design: Participants will have at least 7 clinic visits over 5 months. Participants will have screening and baseline visits. They will have blood tests and tests of their heart function. They will give a sputum sample. Other tests will include: Spirometry: Participants will breathe in and out through a mouthpiece to measure how much air they can hold in their lungs and how hard they can breathe. Diffusion capacity of lungs for carbon monoxide: Participants will breathe in a gas that contains a small amount of carbon monoxide. Then they will breathe through a mouthpiece. This test measures how well oxygen moves from the air into the blood. Resting energy expenditure. Participants will lie still for 30 minutes with a clear dome over their head. This test measures the calories their body burns at rest. 6-minute walk test. Participants will walk at their normal pace for 6 minutes. Their vital signs and blood oxygen levels will be checked. Hymecromone is a tablet taken by mouth. Participants will take 2 tablets every morning and 2 tablets every night for 12 weeks. Tests will be repeated at study visits.