A 39 year old man who was HIV positive and was treated with trimethoprim-sulfamethoxazole for pneumocystis with hypoxaemia. During the acute episode he had a persistent fever of 38 degrees and hypoxaemia with a PaO2 of 65 mm/Hg and bilateral opacities both radiologically and on a CT scan, which were of alveolar type, with bronchograms identical to those observed before the treatment of the pneumocystis. In view of the negative evidence for a respiratory or extra respiratory infection, a surgical biopsy was performed and this revealed lesions of bronchiolitis obliterans with an organising pneumonia (BOOP). After the thoracotomy, there was a spontaneous clinical cure in a few days and radiological clearance in a month. This very rare diagnosis should be added to the list of causes of alveolar pneumopathy with infiltration and fever occurring during the course of an HIV infection.