The frequency of Aspergillus fumigatus isolates from sputum was assessed prospectively during a 22-month period in 156 patients with cystic fibrosis (CF) from one center, and findings were compared to a cross-sectional evaluation of specific IgG and IgA antibodies, occurrence of chronic Pseudomonas aeruginosa infection, and pulmonary function. The prevalence rate for the 22-month period was 40%. Positive A. fumigatus cultures appeared to be independent of the presence or duration of chronic bronchopulmonary Ps. aeruginosa infection, but isolation of A. fumigatus in patients with pseudomonas infection for more than 5 years was associated with notably decreased pulmonary function. Levels of IgG antibodies to a 470,000 daltons A. fumigatus antigen fraction were higher in patients with positive cultures in the observation period than in those without. IgG antibodies to a 25,000-50,000 daltons antigen fraction were directly correlated to A. fumigatus frequency in patients with positive cultures both prior to and during the survey. On the other hand, levels of IgA antibodies to the 470,000 daltons fraction were inversely related to A. fumigatus frequency, suggesting a role of IgA antibodies in the bronchial clearance of aspergilli. Decreased pulmonary function was found to be associated with elevated levels of A. fumigatus antibodies. It is concluded that immune reactions elicited by A. fumigatus may play a role in clearance of the fungus from the airways but also may contribute to lung morbidity in some patients.