Colonization with Aspergillus fumigatus (Af) constitutes a common finding in children with cystic fibrosis (CF). The relationship between sensitization to Af and lung functon (LF) was studied in 118 patients with CF (61 girls and 57 boys; mean age: 14.3 yr; SD: 7 yr). Mean follow up was 2.2 yr. On average, 8.1 (SD: 4.8) LF tests were performed per patient. Measurement of total IgE and specific IgE antibodies to Af, and a skin prick test (SPT) for Af, were done once a year. Thirty-one children (26%) were sensitized to Af. On average, LF parameters were not significantly different in Af-sensitized children than in nonsensitized children. Linear regression analyses were performed, using the repeated measures design. With adjustment for gender, age, height, and weight, sensitization to Af was associated with lower values of FEV1 (beta = -0.209; p < 0.05) and FEF(25-75) (beta = -0.356; p < 0.01). Analysis of different subgroups of sensitization demonstrated the effect on LF only in Af-sensitized patients with elevated total IgE levels, and not in Af-sensitized patients with normal IgE levels. Furthermore, there was evidence for a more rapid decline in LF for Af-sensitized patients with elevated total IgE levels than in those with normal IgE levels. We conclude that sensitization to Af in the presence of increased IgE values is associated with lower LF values in children with CF.