Background: Cystic fibrosis (CF) is the most common autosomal recessive disease in Caucasian population. Allergic bronchopulmonary aspergillosis (ABPA) is one of the severe complications of CF, on which diagnosis is based on symptoms and blood IgE levels. Many techniques of specific IgG levels measures are used, which signification is still unclear. We evaluated evolution of patients who presented a first aspergillosis IgG seroconversion. Methods: Monocentric pediatric case-control study led in Rouen, France. Every patient with a first aspergillosis IgG seroconversion was paired with a seronegative patient. Clinical data, functional respiratory investigations, CT-scan and biologic data were collected a year before (Y _-1), a year after (Y ₊₁) and at the moment of the first aspergillosis seroconversion. Results: 36 cases, paired with 36 controls. Median age was 8. Forced expiratory volume in 1 second was significantly lower at Y ₊₁ (p=0,025) and Vital Capacity was significantly lower at Y ₀ (p= 0.027) in the case-population. More respiratory exacerbations were observed in the case-population (p=0,047). Higher specific IgE against A. fumigatus levels were observed at Y ₀ (p=0,014), Y _-1 (p=0,001) and Y ₊₁ (p=0,04) in the case-population. Total IgG were significantly higher at Y ₀ in the case-population. On the CT-scan, bronchiectasis and pulmonary infiltrates were more important in the case-population (p=0,01 and p=0,003 respectively). Conclusion: Aspergillosis seroconversion is associated with changes of clinical, respiratory functional, biologic and radiologic parameters in CF population. Aspergillosis seroconversion is a milestone in the evolution of CF. A systematic research is needed, to evaluate actions to be taken.