Fifty adult subjects for whom a diagnosis of idiopathic bronchiectasis (excluding those secondary to tuberculosis or hypogammaglobulinaemia) had been confirmed previously were investigated by: questionnaire; blood eosinophil count; sputum culture for Aspergillus fumigatus and eosinophil count; chest radiography; skin-prick tests with several aeroallergens and four preparations of A. fumigatus, including a reference extract; measurement of specific IgE antibodies; precipitin testing and self-crossed immunoelectrophoresis with A. fumigatus. Five subjects were possible cases of allergic bronchopulmonary aspergillosis in whom the condition had been previously misdiagnosed or in whom sensitization to A. fumigatus had occurred after the onset of bronchiectasis. These five subjects had positive immediate skin reactions to A. fumigatus and a history of recurrent pneumonias. Four had a previous history of asthma and the others showed increased bronchial responsiveness to inhaled methacholine. At the time of the survey, A. fumigatus grew in the sputum of one out of five subjects. These subjects had increased levels of specific IgE. Two had precipitins by double diffusion and three subjects were positive on self-crossed immunoelectrophoresis. It is concluded that allergic bronchopulmonary aspergillosis or evidence of sensitization to A. fumigatus can be identified in a significant proportion of adult subjects with so-called idiopathic bronchiectasis.