Pulmonary diseases associated with tissue and/or blood eosinophilia are a heterogeneous group of disorders. Various nosologies have been offered, but this article classifies these syndromes as extrinsic or intrinsic in origin. Some syndromes overlap, but this approach is convenient from the diagnostic standpoint. Inhaled or ingested extrinsic factors, including medications and infectious agents (e.g., parasites, fungi, mycobacteria), may trigger an eosinophilic immune response. This may be mild and self-limited, as in Loffler syndrome. Eosinophilia and pulmonary infiltrates have been reported in patients with AIDS, lymphoma, a variety of inflammatory lung diseases, and collagen vascular diseases. Asthma may manifest with marked eosinophilia, with or without infiltrates.