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Following evidence from randomized controlled trials, patients with bronchiectasis unrelated to cystic fibrosis receive long-term azithromycin to reduce acute respiratory exacerbations. However, the period when azithromycin is effective and which patients are likely to most benefit remain unknown. (i) What is the period after its commencement when azithromycin is most effective? and (ii) Which factors may modify azithromycin effects? A secondary analysis was conducted of our previous randomized controlled trial involving 89 indigenous children with bronchiectasis unrelated to cystic fibrosis. Semi-parametric Poisson regression identified the azithromycin efficacy period. Multivariable Poisson regression identified factors that modify azithromycin effect. Azithromycin was associated with fewer exacerbations per child-week during weeks 4 through 96, with the most effective period observed between weeks 17 and 62. Eleven factors were associated with different azithromycin effects; four were significant at the P <.05 level. Compared with their counterparts, higher reduction in exacerbations was observed in children with nasopharyngeal carriage of bacterial pathogens (incidence rate ratio [IRR] = 0.81 [95% CI, 0.57-1.14] vs 0.29 [0.20-0.44]; P <.001); New Zealand children (IRR = 0.73 [0.51-1.03] vs 0.39 [0.28-0.55]; P =.012); and those with higher weight-for-height z scores (interaction IRR = 0.82 [0.67-0.99]; P =.044). Compared with their counterparts, lower reduction was observed in those born preterm (IRR = 0.41 [0.30-0.55] vs 0.74 [0.49-1.10]; P =.012). Regular azithromycin is best used for at least 17 weeks and up to 62 weeks, as these periods provide maximum benefit for indigenous children with bronchiectasis unrelated to cystic fibrosis. Several factors modified azithromycin benefits; however, these traits need confirmation in larger studies before being adopted into clinical practice. Australian New Zealand Clinical Trials Registry; ACTRN12610000383066. [ABSTRACT FROM AUTHOR]