We describe a case of a patient with idiopathic pulmonary alveolar proteinosis (PAP), who had an elevated serum level of antigranulocyte-macrophage colony stimulating factor (anti-GM-CSF) antibody accompanied by T-cell receptor gene rearrangements in BAL fluid cells. Histopathological examination of the lung excluded lymphoma but revealed PAP and silicosis. There was no detectable serum anti-GM-CSF antibody in 50 outpatients with advanced silicosis who did not have PAP, suggesting that anti-GM-CSF antibody is directly linked to PAP but not to silicosis. We speculate that monoclonal expansion of a T-cell population may play a role in the production of anti-GM-CSF antibody and the development of PAP.