Through gleeding into the lung parenchyma is responsible for morbidity and mortality in patients with leukaemia pulmonary haemorrhage is seldom diagnosed during life. We diagnosed occult pulmonary haemorrhage in five leukaemic patients with unexplained infiltrates on chest roetgenograms by examining alveolar macrophages retrieved by bronchopulmonary lavage. Macrophage haemosiderin content was greatly increased in the patients with pulmonary haemorrhage as compared to normal and thrombocytopenic control subjects. Haemoglobin and intact erythrocytes in alveolar macrophages were taken as evidence of recent haemorrhage. Intrapulmonary bleeding may occur often in patients with leukaemia, and bronchopulmonary lavage offers a safe approach to diagnosis and allows for concomitant identification of pulmonary infection.