Hypereosinophilic syndromes with pulmonary involvement can be idiopathic or associated with a number of conditions such as parasitic infections, drug reactions, and so on. In addition, they can also be acute or chronic depending on the clinical course. It is important to realize that eosinophils are not seen in lungs of normal individuals, so their presence in transbronchial biopsies should alert the pathologist to the possibility of eosinophilic pneumonia or other eosinophil-associated lesions. Pulmonary eosinophilia may be seen in the absence of peripheral blood eosinophilia. The transbronchial biopsy may show a constellation of findings including intra-alveolar eosinophils, macrophages, and an amorphous proteinaceous exudate. When fibrin accompanies the eosinophils, the possibility of acute eosinophilic pneumonia should be considered. As is common in dense eosinophilic infiltrates, Charcot-Leyden crystals may be found. Eosinophils infiltrate the interstitium in about 80% of the cases, and foci of organizing pneumonia may be present. Eosinophilic abscesses with foci of necrosis and palisading of histiocytes are classically described in chronic eosinophilic pneumonia; however, a transbronchial biopsy may not show all of the described features. Although the presence of vasculitis has been described in eosinophilic pneumonias, the pathologist should document this finding, and the possibility of Churg-Strauss vasculitis should be ruled out clinically. Prior treatment with steroids may mask the eosinophilia, and the biopsy may only show accumulation of intra-alveolar macrophages. Some series have reported diagnostic changes of eosinophilic pneumonia by transbronchial biopsy in 64% of patients with peripheral eosinophilia and pulmonary infiltrates.