Pulmonary alveolar proteinosis, also termed alveolar lipoproteinosis, represents an autoimmune defect of the alveolar macrophages to remove surfactant with the development of autoantibodies that impair macrophage function by neutralizing granulocyte simulating factor. Alveolar proteinosis is most often seen in middle-age patients and more often in men than women. It has also been reported in children with underlying enzyme defects, patients exposed to heavy dust, and in patients with underlying malignancies such as leukemia or lymphoma or in immunodeficiency states. Treatment is most usually by bronchiolar lavage; however, spontaneous remission can also occur. On a transbronchial biopsy, alveolar proteinosis is seen as a uniform expansion of the intra-alveolar spaces with an eosinophilic slightly granular amorphous material often also containing globular clumps of eosinophilic material. Typically, scattered macrophages and scattered cholesterol clefts are also present. No inflammatory component or red blood cells are seen within the material. The interstitium is not expanded and shows no inflammation, although longstanding chronic cases may develop interstitial fibrosis and inflammation. Periodic acid-Schiff (PAS) with diastase stains positive, confirming the presence of surfactant apoprotein that contains glycogen. Pulmonary alveolar proteinosis may be associated with infections including Nocardia, mycobacteria, fungus, or virus.