Bronchoalveolar lavage (BAL) is a minimally invasive method of medical diagnosis that involves injecting a sterile physiological fluid solution at 37°C or a mucolytic agent into the bronchi and alveoli of the lungs. This fluid is then collected and used to perform certain tests to look for infections or other pathologies. Bronchoalveolar lavage (BAL) is used to sample the alveolar space. The alveolar space may contain immune and inflammatory cells, cytokines, enzymes, infectious and environmental agents.

Bronchoalveolar lavage represents a method of exploration of the alveolar territories, i.e. the "deep lung". By studying cell populations, it provides interesting information for the diagnosis and monitoring of a certain number of diffuse respiratory diseases (sarcoidosis, pulmonary fibrosis, immunoallergic pneumopathy). Moreover, even with the knowledge of the risks and possible pain for the patient, this examination is very cost-effective (in terms of benefit/risk) for establishing a differential diagnosis.