Seven to ten percent of patients with cystic fibrosis had serum antibodies to the catalase antigen of Aspergillus fumigatus in three cross-sectional surveys between 1977 and 1984. A total of 208 patients participated at least once, and the cumulated frequency of catalase antibodies in 94 patients included in all three surveys was 16%. The titre range was 1 to 16. The prevalence rate of Aspergillus fumigatus in sputum was 50% for a 2.5-year observation period. Catalase antibodies were strongly associated with the occurrence of Aspergillus fumigatus in sputum (p = 0.003), and the microorganism was more numerous in colonized patients with catalase antibodies than in those without such antibodies (p = 0.004). Patients with Aspergillus fumigatus in sputum and a positive catalase antibody test tended to have an adverse development as regards lung function compared to both carriers without antibodies and non-carriers. The observed differences could not, however, be related to different rates of chronic Pseudomonas aeruginosa infection.