Bronchoalveolar lavage (BAL) can be a very useful tool in the diagnosis of interstitial lung disease, but BAL must be performed properly and the retrieved BAL fluid adequately processed and analyzed to allow accurate conclusions to be drawn from BAL analysis. A differential cell count of nucleated immune cells can show cell patterns that suggest or support certain diagnoses, and other testing (stains and cultures for infectious pathogens, malignant cell cytology) can be performed on BAL fluid that can also aid in diagnosis. When combined with the results of a careful history, physical examination, thoracic imaging, and other pertinent laboratory testing, the BAL analysis may allow a confident diagnosis of a specific interstitial lung disease to be made without proceeding to more invasive testing (e.g., surgical lung biopsy) that is associated with increased risk of complications.