Pulmonary alveolar proteinosis (PAP) is a rare lung disease characterized by filling of the distal alveoli with endogenous proteinaceous or floccular material derived from surfactant phospholipids and protein components. This disease presents in two forms: primary (idiopathic) or secondary (resulting from infection, hematologic malignancy(ies) or inhalation of a foreign substance).

The incidence of PAP is estimated to be 1:100,000. Males are affected more often by a ratio of 4:1.

The etiology of primary PAP is still not known. Secondary PAP is associated with several processes, which lends credence to a causal relationship. The associated processes include inhalation of inorganic/mineral dusts (e.g., silica, titanium oxide, aluminum) or organic materials (e.g., insecticides); hematologic malignancy(ies); myeloid, lysinuric protein intolerance, or HIV infection.