Chronic use of azithromycin has been associated with increased macrolide resistance in recognised CF pathogens. The aims of this study were to determine 1. macrolide resistance amongst other members of the CF airway microbiota 2. if resistance is associated with current patient prescription of azithromycin. The susceptibility (MIC) of CF respiratory isolates (n = 26: Streptococcus, n = 8; Rothia, n = 5; Veillonella, n = 5; Prevotella, n = 4; Actinomyces, n = 3; Fusobacterium, n = 1) to azithromycin, clarithromycin and erythromycin was determined by Etest®. Isolates were separated into two groups depending on whether they were isolated from CF patients currently prescribed or not currently prescribed azithromycin and macrolide susceptibility was compared between the groups using the Mann–Whitney test. High-level macrolide resistance (>256 mg/mL) was common among the isolates tested: azithromycin, n = 10/26 (38%); clarithromycin, n = 6/26 (23%); erythromycin, n = 11/26 (42%). Isolates from CF patients currently prescribed azithromycin (n = 9) had significantly higher MICs for the three macrolide antibiotics compared to isolates from patients not prescribed azithromycin (n = 17) [Table]. Table Azithromycin prescription versus susceptibility MIC50 (mg/mL) P Current prescription of azithromycin (n = 9) No current prescription of azithromycin (n = 17) Azithromycin >256 2 0.005 Clarithromycin 47 0.5 0.001 Erythromycin >256 4 0.01 Resistance to three different macrolide antibiotics was common in bacteria present in the CF airway microbiota and was associated with current azithromycin prescription in CF patients. Work supported by a Wellcome Trust (FA) Vacation Scholarship.