Although long-term azithromycin decreases exacerbation frequency in bronchiectasis, increased macrolide resistance is concerning. We investigated macrolide resistance determinants in a secondary analysis of a multicenter randomized controlled trial. Indigenous Australian children living in remote regions and urban New Zealand Māori and Pacific Islander children with bronchiectasis were randomized to weekly azithromycin (30 mg/kg) or placebo for up to 24 months and followed post-intervention for up to 12 months. Nurses administered and recorded medications given and collected nasopharyngeal swabs 3–6 monthly for culture and antimicrobial susceptibility testing. Nasopharyngeal carriage of Haemophilus influenzae and Moraxella catarrhalis was significantly lower in azithromycin compared to placebo groups, while macrolide-resistant Streptococcus pneumoniae and Staphylococcus aureus carriage was significantly higher. Australian children, compared to New Zealand children, had higher carriage overall, significantly higher carriage of macrolide-resistant bacteria at baseline (16/38 versus 2/40 children) and during the intervention (69/152 versus 22/239 swabs), and lower mean adherence to study medication (63 % versus 92 %). Adherence ≥70 % (versus <70 %) in the Australian azithromycin group was associated with lower carriage of any pathogen [odds ratio (OR) 0.19, 95 % confidence interval (CI) 0.07–0.53] and fewer macrolide-resistant pathogens (OR 0.34, 95 % CI 0.14–0.81). Post-intervention (median 6 months), macrolide resistance in S. pneumoniae declined significantly in the azithromycin group, from 79 % (11/14) to 7 % (1/14) of positive swabs, but S. aureus strains remained 100 % macrolide resistant. Azithromycin treatment, the Australian remote setting, and adherence <70 % were significant independent determinants of macrolide resistance in children with bronchiectasis. Adherence to treatment may limit macrolide resistance by suppressing carriage.