Twenty-eight selected patients with pulmonary aspergilloma and many with respiratory symptoms were investigated. Their atopic status was evaluated and immunological reactivity to extracts of A. fumigatus studied by means of skin, serological and inhalation challenge tests. The findings suggested that immunological mechanisms may have played a role in producing symptoms in 75% of these patients. Based on these findings, corticosteroids may have a place in treatment of some patients with pulmonary aspergilloma. Care in the use of corticosteroids is recommended because of the prevalence of bacterial infection. Suggestions are offered for selecting patients likely to respond to corticosteroid drugs.