Pulmonary alveolar proteinosis (PAP) is a rare disorder characterized by the presence of eosinophilic material in the alveoli. Recent data suggest that PAP is due to a deficiency of granulocyte-macrophage colony-stimulating factor (GM-CSF) or receptors for GM-CSF. Patients generally present with cough or dyspnea, but physical examination findings are nonspecific. The classic chest radiographic pattern is a bat wing appearance, although various radiographic patterns have been described. Either transbronchial or surgical biopsy can confirm the diagnosis. When indicated for dyspnea or hypoxemia, treatment consists of whole lung lavage. Ongoing studies are evaluating the efficacy of GM-CSF injection as an alternative therapy. Although the prognosis in PAP is generally good, relapse is common.