The prevalence of allergic bronchopulmonary aspergillosis (ABPA) in cystic fibrosis (CF) patients is difficult to determine because the data in the literature are not homogeneous or comparable. ABPA and CF have similar clinical symptoms which make diagnosis difficult and underestimate the real dimensions of the problem. We conducted an epidemiological study on 3089 Italian CF patients to determine the prevalence of ABPA in Italy and verify the percentage of positive tests in the employed diagnostic criteria. Our results indicate that the prevalence of ABPA in Italian CF patients is 6.18%, mainly in adolescents and young adults. ABPA is diagnosed using clinical symptoms (presence of episodic bronchial obstructions or typical radiographic features) and on the basis of other criteria which can only be partially fulfilled in paediatric patients. Among the diagnostic tests the most sensitive are the total IgE (84.5%), specific IgE anti-Aspergillus fumigatus (81.6%) and the prick test (68.3%). In the absence of clinical symptoms and gold standard diagnostic tests, serological positivity and/or the skin test are not sufficient evidence to confirm the presence of ABPA.