A 57-year-old man with myelodysplastic syndrome developed dyspnea on exertion in September 1993. Chest X-ray film showed diffuse infiltrative shadows in the middle and lower lung fields on both sides. The infiltrates were prominent in the perihilar regions but less so in the periphery, consistent with a "bat wing pattern." Milky fluid was obtained by bronchoalveolar lavage. The specimens obtained from transbronchial lung biopsy were compatible with the pathological findings of pulmonary alveolar proteinosis with amorphous periodic acid-Sciff-positive materials in the alveolar spaces. Impaired function of alveolar macrophages was demonstrated from their abnormally low phagocytosis of FITC-labeled latex beads. Whole lung lavage under general anesthesia was done twice, but the patient died because of progression of respiratory failure.