A 60-year-old man with a myeloproliferative syndrome and extramedullary hematopoiesis had progressive respiratory and cardiac insufficiency during the previous 18 months, with advancing interstitial pulmonary disease on chest x-ray. During analysis of his respiratory disease, results of a transbronchial biopsy showed interstitial involvement with increased numbers of megakaryocytes and other panhematopoietic staining elements. Results of a bone marrow scan demonstrated diffuse replacement of pulmonary interstitium with bone marrow, as a component of known ongoing extramedullary hematopoiesis.