In predisposed patients, allergic bronchopulmonary aspergillosis (ABPA) can arise from aspergillus bronchial colonization. We report the case of a young woman who presented with a right basal pneumonia, ground glass opacities and mediastinal adenopathies on CT scan. Biological, radiological and clinical criteria, as well as an history of childhood asthma, allowed the initial diagnosis of ABPA. However, the unusual coexistence of an additional infection with Pseudomonas Aeruginosa evoked the diagnosis of cystic fibrosis, confirmed by a sweat test and genetic analysis. Under corticosteroid and antifungal therapy and antibiotics, the clinical and radiological evolution was favourable but immuno-allergic sensitisation persisted. The ABPA-cystic fibrosis association is not rare with an estimated prevalence of 2% to 11% according to previous studies. This variability is partly explained by the difficulty of the diagnosis due to confounding clinical, radiological, and biological signs between ABPA and cystic fibrosis. Many predictive development factors of ABPA in the context of cystic fibrosis have been reported, including respiratory function, personal or familial atopy, colonization with Pseudomonas Aeruginosa and age. As in non cystic fibrosis patients, the treatment requires systemic corticotherapy and itraconazole. ABPA is still often under diagnosed and should be evoked in the context of cystic fibrosis.