Aspergillus colonization is a common phenomenon in adult cystic fibrosis (CF) patients. The clinical significance of Aspergillus for the pathogenesis of CF lung disease remains unclear and factors predisposing to such colonization are still completely unknown. We investigated the prevalence of Aspergillus colonization in 104 adult CF patients who attended our outpatient clinic in 1997. With respect to demographic and clinical data, and antibiotic therapy received, we further examined which factors were associated with Aspergillus colonization in these patients. Repeated investigations of CF sputum samples revealed Aspergillus species in 43/104 (41.3%; 95% confidence interval 30.2-52.5%) of the patients. We found no significant relationship between Aspergillus colonization and age (P > 0.4), gender (P = 0.4), colonization with pseudomonas species (P > 0.6), lower lung function values (P > 0.9), or worse chest radiography (P > 0.1). Surprisingly, the prevalence of Aspergillus colonization was higher in CF patients receiving prophylactic antibiotic therapy (oral antibiotics: P = 0.05; inhalative antibiotics: P = 0.035; both antibiotics: P = 0.048). Prophylactic antibiotics are widely used to eradicate or decrease chronic bronchopulmonary infection in CF. Our results indicate that long-term antibiotic therapy may predispose CF patients to Aspergillus colonization.