Abstract: Bronchial alveolar lavage fluid （BAL） plays a pivotal role in the diagnosis of lung diseases. Detection of a large amount of characteristic foamy cells in BAL is rarely encountered. In this article, a large quantity of foamy cells was detected in the BAL of 4 cases. They were diagnosed with pulmonary adenocarcinoma combined with Trichinella infection, pulmonary adenocarcinoma, pulmonary cryptococcal infection, SLE with antiphospholipid antibody syndrome and pulmonary embolism. The diagnosis and treatment of these patients prompted that the range of clinical diagnosis could be rapidly narrowed if a massive amount of foamy cells was detected in the BAL. Moreover, the diagnosis can be confirmed by complete rheumatism immunity-related examination, enhanced CT scan of the lung, Cryptococcus antigen or parasite-related examination, tumor screening, inquiry of medical history of drug use and environmental exposure, etc.