Acute respiratory failure and the "acute respiratory distress syndrome" (ARDS) are frequent medical conditions in critically ill patients. Various causes can potentially result in the development of ARDS. Two cases are presented, in which malignant diseases were identified as causes of the respiratory failure. The first patient was diagnosed with an acute myeloic leukemia M5 (FAB). In the second patient, lung histology revealed an adenocarcinoma of the lung. These case reports show that in addition to the classical causes of ARDS, specific disease entities can mimic this form of respiratory failure. Beside solid cancers and lymphomas, acute and progressive forms of inflammatory, parenchymal lung diseases (such as acute interstitial pneumonitis, acute eosinophilic pneumonia, diffuse alveolar hemorrhagia, and acute hypersensitivity pneumonitis) can manifest with this picture. As a consequence, the diagnostic workup of respiratory failure of unknown cause should include these entities.