Many workers have suggested that exposure to an environment rich in Aspergillus spores is aetiologically significant in allergic bronchopulmonary aspergillosis. This hypothesis has been investigated by means of an environmental survey in which eight urban dwelling patients with this disease and five atopic control patients took part. Enquiries were made about contact with known sources of Aspergillus fumigatus and air spora studies were conducted in the home. Findings were similar for both groups. No high spore concentrations were recorded in the homes of either group. Patients with allergic bronchopulmonary aspergillosis had not been more exposed to potentially rich sources of A. fumigatus than the atopic control patients. The view is expressed that specific host susceptibility is more important in the pathogenesis of allergic bronchopulmonary aspergillosis than environmental factors. Once a patient is sensitized however minor increases in spore concentration can cause symptomatic disease producing the recognized winter exacerbations of allergic bronchopulmonary aspergillosis. Possible mechanisms allowing the development of the disease in the presence of low spore concentrations are discussed.