We analyzed phospholipids, especially the acidic phospholipids, i.e. phosphatidylglycerol (PG) and phosphatidylinositol (PI), of bronchoalveolar lavage (BAL) fluids from patients with pulmonary alveolar proteinosis. A significant decrease in PG and a concomitant increase in PI were found in the phospholipid of the patient's BAL fluids compared to that of normal subjects. Thus, the PG to PI ratio was significantly decreased in pulmonary alveolar proteinosis. These changes in the acidic phospholipids in BAL fluids of patients with pulmonary alveolar proteinosis appear to indicate that a switch-over in the synthesis of both acidic phospholipids from a same precursor CDP-diacylglycerol (CDP-DG) in alveolar Type II cells occurs in a such diseased state. However, since the molecular species profiles of PG and PI in the patient's BAL fluids were distinctly different from each other, the PG and PI present in the alveoli of the patients appeared to be derived from different pools of CDP-DG in alveolar Type II cells.