Aspergillus fumigatus, a widely distributed spore-bearing fungus, is commonly grown in sputum cultures of patients with cystic fibrosis (CF). A fumigatus may cause allergic bronchopulmonary aspergillosis (ABPA), a complex condition that leads to worsening of airway inflammation and progressive damage and is diagnosed by specific criteria. In this report, we present six CF patients with respiratory deterioration that did not respond to appropriate antibiotic treatment. All had had A fumigatus in sputum cultures but did not fulfill the criteria of ABPA. Treatment with antifungal agents was followed by improvement in clinical condition. We suggest that in patients with CF, A fumigatus should be considered as a pathogen that may directly cause respiratory exacerbations. Antifungal therapy should be considered when deteriorating respiratory function is not responding to antibacterial therapy and A fumigatus is growing in sputum cultures.