BOOP and COP are essentially the same condition and represent one of many ways in which the lung may respond to an inflammatory stimulus. Some underlying causes of BOOP have been identified but in many cases no cause can be found. The clinical and radiological features are of a pneumonic illness that responds to corticosteroids rather than antibiotics, but as milder cases are being identified the clinical spectrum is widening. Most cases can be confidently diagnosed only by open lung biopsy, but bacteriological lavage and transbronchial biopsy followed by a trial of steroids may sometimes be considered.