Bonchoalveolar lavage is a safe and poorly invasive tool with a great diagnostic value particularly in diffuse infiltrative pulmonary diseases (IPD). In specific instances, it allows for a definite diagnosis (alveolar hemorrhage; alveolar proteinosis, lipidoses, infiltrative malignant diseases, opportunistic infections), obviating the need to perform more invasive diagnostic procedures like video-assisted surgical biopsy. In inflammatory IPD, either idiopathic, diagnostic or associated with inhaled antigens or with collagen vascular diseases for instance, it represents a crucial orientation diagnostic tool, considerably narrowing the spectrum of potential differential diagnosis.