Allergic bronchopulmonary aspergillosis (ABPA) is a severe disease inducing bronchopulmonary anatomic lesions which complicate those already present in patients with cystic fibrosis. The frequency of this association is estimated between 0.6 and 10% according to American studies. Diagnosis of ABPA is difficult, as findings such as sibilant rales, pulmonary infiltrates, bronchiectasies, anti-aspergillus precipitins may be present as single features in patients with cystic fibrosis. Thus it is important to ascertain the diagnosis as oral corticosteroid treatment is the only one able to prevent evolution towards bronchiectasies and pulmonary fibrosis.