Patients with preleukemic myeloid neoplasia can develop nonhematologic disease. Five patients with the myelodysplastic syndrome presented with interstitial lung disease that heralded acute leukemia in 3. Chest radiographs showed diffuse interstitial opacities, and the lung biopsies showed diffuse cellular interstitial and fibrosing pneumonitis with prominent alveolar filling by macrophages. There was no evidence to support a drug-induced or infectious etiology, and all cases lacked an identifiable leukemic infiltration. The inflammatory infiltrates and fibrosis were analyzed morphometrically, and this revealed a trend toward an indirect correlation between both CD68 cells and MPO-positive inflammatory cells and pulmonary fibrosis. We conclude that preleukemic myeloid neoplasia can be associated with an interstitial pneumonitis with histopathologic features that are distinguishable from both leukemic infiltration and "usual" nonspecific interstitial pneumonia.