Monozygotic twins are reported; both had bronchial asthma with type I hypersensitivity to Aspergillus fumigatus but only one had type III hypersensitivity, together with pulmonary infiltrations compatible with the diagnosis of allergic bronchopulmonary aspergillosis. It is suggested that non-inherited factors other than the intensity of exposure to antigen may be important in determining the development of allergic bronchopulmonary aspergillosis.