The incidence of serum IgE antibodies to several species of aspergillus and the incidence of serum precipitins to these species and to Candida albicans were determined in 22 children with cystic fibrosis, 17 children with bronchial asthma, and 18 control subjects. IgE antibodies were found in 50 per cent of the patients with cystic fibrosis and in 29 per cent of the patients with bronchial asthma; none was found in the control subjects. Precipitins to aspergillus species were present in 59 per cent of the patients with cystic fibrosis, 35 per cent of the patients with bronchial asthma, and 11 per cent of the control subjects. Greater differences were noted when the total number of precipitin bands in each group was compared. There was a trend for the more severe cases of cystic fibrosis (based on the Shwachman-Kulczycki score) to have a greater incidence of precipitins and a lower frequency of IgE antibodies to aspergillus than the milder cases. Candida precipitins were found more frequently in patients with cystic fibrosis (45 per cent) than in patients with bronchial asthma (18 per cent); none was found in the control group. The potential role of fungal hypersensitivity in cystic fibrosis and the therapeutic implications are discussed in light of these findings.