Objectives The aim of the study is to compare data from an ELISA ( A. fumigatus – Bordier Affinity Products SA, CH) and a Western Blot kit (WB Aspergillus IgG – LDBio Diagnostics, France) with double diffusion (DD) method in agar gel (Ouchterlony). Methods 91 sera from CF pediatric and adult patients followed at our hospital were tested retrospectively. DD, a semi-quantitative method, was performed using somatic and metabolic extracts of A. fumigatus (BioRad and FSK, Microgen Bioproducts Ltd). The interpretation is based on the absence/presence and number of precipitation arcs. ELISA and WB were performed according to manufacturer's instructions. Conclusion The 91 sera were classified, according to the DD results, in negative (29), positive (49) and uncertain (13). Six out of the 29 negative sera were positive by ELISA, two out of the 6 were also positive to the WB, both belonging to patients with positive cultures. Notably, one of the two patients was affected by allergic bronchopulmonary aspergillosis. Among the 49 DD positive sera, 3 belonged to patients with positive culture but with stable clinical conditions and were negative for ELISA and WB. Statistical analysis of the data (excluding the doubtful cases) suggests that the distribution of inconsistent outcomes between the ELISA vs DD is compatible with the hypothesis of random variability (p = 0.508). The greatest number of positive results by ELISA compared to WB is not compatible with the hypothesis of random variability (p = 0.022). These assessments suggest that the replacement of DD with the ELISA test does not involve a deviation between the results obtained and that ELISA test confirms its validity as screening test.