Abstract Poster session 1, September 21, 2022, 12:30 PM - 1:30 PM Background Aspergillus fumigatus, a globally distributed opportunistic pathogen, is the main cause of invasive aspergillosis, especially in immunocompromised patients with high mortality. The emergence of azole-resistant A. fumigatus isolates has been a significant concern worldwide and an important clinical problem. Objectives We aim to determine the presence of variants in a large collection of clinical A. fumigatus isolates from the Netherlands, if the number of variants increased over time and if the presence of additional short nucleotide polymorphisms (SNPs) or tandem repeats (TR) variations impacted on the triazole phenotype. Methods The Radboud University Medical Center has collected 11 813 clinical A. fumigatus isolates since 1994. The collection includes isolates cultured from patients admitted to our own center, isolates sent from other hospitals for identification and in vitro susceptibility testing, and isolates sent from five university medical centers and five teaching hospitals that contribute to the national Aspergillus resistance surveillance. The genotypes were detected by Cyp51A Sanger sequencing. All isolates were subjected to in vitro susceptibility testing using the EUCAST microdilution reference method. Minimal inhibitory concentrations (MICs) were determined for itraconazole, voriconazole, posaconazole, in all isolates and for isavuconazole in isolates cultured in 2015 and thereafter. Results In total, 1826 A. fumigatus isolates harbored azole-resistant mutations in the Cyp51A-gene with 92 genotypes. Tandem Repeat-associated resistance genotypes accounted for 55.43% of the variants and were involved in 1728 isolates (94.63%). TR34/L98H and TR46/Y121F/T289A resistance mutations remained dominant, and increasingly additional SNPs in the Cyp51A-gene or changes to the gene promoter were observed. The G448S mutation was relatively common and present in various genetic backgrounds. This SNP was most often found in isolates ...