A 65-year-old woman with dermatomyositis for which she had been treated for ten years with prednisone (latterly 15 mg daily) suddenly experienced severe pain in the left thoracolumbar region. Cardiovascular, pulmonary and vertebral causes of the pain were excluded. But serological tests indicated inflammatory disease and the haemoglobin concentration was low (10.4 g/dl). Left pleural effusions were repeatedly aspirated and some haemorrhagic fluid obtained (haematocrit 0.31 in blood, 0.28 in the pleural effusion). Five days after admission her cardiovascular status became unstable and she developed respiratory failure (haemoglobin 7.6 g/dl). Chest radiograph showed increased pleural effusion. Subsequent thoracotomy revealed a left coagulothorax which was removed and flushed. During this procedure severe bleeding occurred from a covered perforation of the descending aorta, 1.5 x 1.5 cm in size. Although the aortic wall was thin, there was no aneurysm but arteriosclerotic changes and an external erosion near an abscessing mediastinitis, originating from a chronic purulent pleuritis and bronchopneumonia. The severe blood loss caused circulatory failure from which the patient could not be resuscitated.