A 5.5-year survey revealed 9 patients with allergic bronchopulmonary aspergillosis (ABPA) and 1 patient with a highly probable diagnosis among 200 Danish patients with cystic fibrosis (CF). The incidence of ABPA was 0.9 (0.4-1.7, 95% confidence limits) per 100 patients per year. All the patients with definite or probable ABPA had serum antibodies to the Aspergillus fumigatus catalase antigen, and in 5 patients the appearance of catalase antibodies during the incipient phase of ABPA was documented. 29 patients without ABPA also had catalase antibodies, but in lower levels than ABPA patients (p = 0.006). The 6-month prevalence rate of A. fumigatus in sputum was 80 and 72% in the 2 groups, respectively. The finding of catalase antibodies in some CF patients without ABPA may indicate the occurrence of a symptom-poor form of pulmonary aspergillosis.