Pulmonary alveolar proteinosis (PAP) is a rare disease characterized by the accumulation of phospholipids and surfactant proteins in the lung. The central role for granulocyte-macrophage colony-stimulating factor (GM-CSF) in surfactant homeostasis has been established in mice lacking the GM-CSF gene, which results in murine pulmonary alveolar proteinosis. No GM-CSF gene defect has been defined in adult patients with idiopathic PAP. Previous studies indicated that the human disease differs from the murine model by the presence of circulating, neutralizing autoantibodies against GM-CSF. Therefore, the final common pathway between the GM-CSF knockout and human PAP appears to be the deficiency of functionally active GM-CSF. In the present study, all patients with idiopathic PAP were found to have systemic and localized antibodies against GM-CSF. Anti-GM-CSF titers were a specific and sensitive marker for PAP. In addition, we present data showing that the absence of active GM-CSF is associated with enhanced levels of macrophage colony-stimulating factor, monocyte chemoattractant protein-1, and interleukin-8. These studies confirm and strengthen previous studies and support the concept that adult idiopathic PAP is an autoimmune disease defined by the presence of anti-GM-CSF. Further, using anti-GM-CSF as an indicator of pulmonary alveolar proteinosis may avoid the use of more invasive means of evaluating patients with pulmonary disease characterized by alveolar infiltrates.