In 10-40% of the patients interstitial pneumonias develop after bone marrow transplantation. The identification of causative agents is often problematic, so that particularly clinical but also blood gas-analytic findings allow the making of a diagnosis. The typical X-ray-morphological changes of the lung (particularly symmetrical, fine-reticular, interstitial multiplications of markings and fine-patchy, striated, partly confluent shadows), which are represented with the help of three instances of cases, confirm the diagnosis, are, however, often found only relatively late. Further diagnostic methods such as the computer tomography or the bronchoscopy, connected with lung biopsy and bronchoalveolar lavage, at present do not play an essential role at least in the early phase after the transplantation of the bone marrow.