We report here our experience of secondary pulmonary alveolar proteinosis (PAP) in patients with hematologic malignancies. The diagnosis of PAP was made by bronchoalveolar lavage (BAL) and based on the identification of periodic acid-Schiff-positive proteinaceous material with the characteristic ultrastructural pattern. Ten patients with leukemia and secondary PAP are described. Three patients had received bone marrow transplants. Data obtained from sequential BAL have shown that at least four of them--all of them achieving complete remission or recovery from neutropenia after bone marrow transplantation--had reversible PAP, and we emphasize this potential reversibility. Furthermore, in order to estimate the frequency of PAP in hematologic patients, we retrospectively studied 113 episodes of pneumonia occurring in our department over a 2-yr period. The incidence of secondary PAP in patients with pulmonary symptoms was so estimated at 5.3% among all the hematologic population, and to 10% in patients with myeloid disorders. This report (1) confirms that BAL is an accurate way to diagnose PAP in immunocompromised hosts, (2) emphasizes that PAP is not an unusual cause of respiratory failure in this population and that it is strongly associated with myeloid disorders, and (3) shows that secondary PAP is potentially reversible, especially if complete remission of the underlying disease is achieved.