Bronchiolitis obliterans syndrome (BOS) is an important complication after hematopoietic SCT (HSCT). Recent observations suggested that azithromycin might improve lung function in BOS after HSCT. We conducted a randomized double-blinded placebo-controlled study on azithromycin in patients with BOS after HSCT. The treatment group (n=10) received oral azithromycin 250 mg daily while the control group (n=12) received placebo daily for 12 weeks. Respiratory symptoms were assessed by the St George Respiratory Questionnaires and spirometry at baseline (drug commencement), 1, 2, 3 (drug cessation) and 4 months (1 month after drug cessation). There was no significant difference in the baseline demographic characteristics between the treatment and the control groups in age, gender, time from HSCT to BOS, time since diagnosis of BOS, chronic GVHD, baseline respiratory symptom scores and baseline forced expiratory volume in 1 s (FEV(1)). Throughout and after 3 months of treatment, there were no significant changes in respiratory symptom scores and FEV(1) measurements between the treatment and the control groups. In conclusion, there was no significant benefit of 3 months of oral azithromycin on the respiratory symptoms and lung function in patients with relatively late BOS after HSCT in this randomized placebo-controlled study.