Bronchiolitis obliterans is an anatomical lesion with multiple aetiologies. In the lung transplant patient the pure forms of bronchiolitis obliterans are probably the consequence of a process of chronic rejection; in fact necropsy tissue or lungs removed which have been transplanted show that the lesions of bronchiolitis obliterans are often associated with parenchymal disorders, vascular and proximal bronchial disease, which are sequelae of phenomena of rejection or infection. The effect of bronchiolitis obliterans on lung function is constant; this may appear progressively or in stages. Increasing immunosuppressive treatment may arrest the progress. This rarely occurs and the development of respiratory failure tends to be the rule. It is exceptional to achieve the diagnosis of bronchiolitis obliterans from the examination of a transbronchial biopsy. It is a combination of features, both clinical and respiratory function, negative bacteriology and virological investigations as well as the absence of any efficacy of conventional treatment for rejection which leads to the diagnosis. In certain cases the question of a pulmonary re-transplantation is raised.