Reports that the relative composition of surfactant phospholipids is normal in PAP patients20 and Csf2−/− mice21, 22 led to a widely-held belief that surfactant accumulation in PAP is caused by impaired catabolism of phospholipids in alveolar macrophages15, however, no such mechanism has been found. Here, CSF2 is linked to pulmonary alveolar proteinosis.