Aspergilli are widely distributed in nature and there is abundant opportunity to acquire infection. The fungus is usually only locally invasive in healthy tissues. Its culture from sputum on one occasion may have no significance. Its growth in the bronchial tree of asthmatics may produce sensitization with aggravation of the asthma and bronchial plugging. Multiplication in dead or damaged lung tissue such as infarcts is not uncommon. The fungus may also colonize pre-existing lung cavities, especially those remaining after treatment of tuberculosis. The ball of fungus produces a characteristic radiological picture, particularly on tomograms, which is usually sufficient to make the diagnosis. Hemoptysis is common. Inaction may be preferable to excision, especially in a patient with impaired lung function. Seven cases have been described to illustrate some of the features and natural history of the condition. The mounting number of sterile cavities left in lungs has increased the number of mycetomas which have developed.