Acute lymphoblastic leukaemia is the most common malignancy in childhood. This disease and its associated therapy may lead to specific life-threatening complications if general anaesthesia has to be carried out. The case of a 14-year-old boy suffering from aspergillosis because of immunosuppression in the course of chemotherapy is reported. Due to a cerebral round lesion an open biopsy was required. After induction of anaesthesia, severe pulmonary obstruction developed. After exchange of the endotracheal tube a coagulum-like foreign body interspersed with Aspergillus hyphae obstructing the distal aperture in a valve-like manner could be recovered. The resulting unilateral tension pneumothorax had to be relieved with a closed pleural drainage. With reference to this as yet unreported life-threatening complication of pulmonary aspergillosis, the appropriate preparation and conduction of general anaesthesia are discussed.