Invasive pulmonary aspergillosis (API) is a necrotising pneumonia generally occurring in profoundly immunodepressed subjects. These observations were based on four patients in the intensive care unit, suffering from chronic respiratory failure (IRC), without profound immunodepression. After a pathophysiological and clinical review, a focus on the diagnostic methods permits one to stress on the reliability, in this type of patient, of the evidence from direct examination of aspergillus filaments in the bronchoalveolar lavage (LBA) or protected bronchial brushings, taking account of the weak value of routine culture of spit or bronchial aspiration in IRC in whom patients are frequently colonised. These four cases permit one to discuss the factors which predispose to the development of API outside the usual immune suppression: IRC itself, by the disorder of mucociliary function, which it leads to; repeated antibiotic therapy which destabilises the saprophytic flora; viral infections which would be responsible for transitory immunodepression. But it is above all steroid therapy which seems to be the major factor favouring the development of API without producing profound immunodepression but probably because it inhibits phagocytosis of aspergillus spores. In these circumstances it is necessary to make an early diagnosis and to use fibre optic bronchoscopy with protected sampling and bronchoalveolar lavage with a complete microbiological. Only early treatment allows one to contemplate a cure.