We present the case of a patient with acute myeloid leukemia and secondary Pulmonary Alveolar Proteinosis (PAP), which is an underestimated cause of a persistent pulmonary infiltrate in patients with hematologic malignancies often accompanied by neutropenia due to therapy. Diagnosis is established by performing Periodic Acid-Schiff reaction (PAS) stains and transmission electron microscopy (EM) on bronchoalveolar lavage (BAL) fluid. We wish to stress the importance of the early recognition of PAP, which is potentially reversible, and routinely performing PAS staining on BAL fluid in patients with a hematologic disease especially myeloid disorders and a persistent lung infiltrate.