Systemic steroids are the standard treatment for bronchiolitis obliterans syndrome (BOS) after allogeneic hematopoietic stem cell transplantation (HSCT) despite their poor efficacy and disabling side effects. To evaluate the effectiveness and tolerance of budesonide/formoterol as an alternative treatment for BOS after HSCT. In this randomized, double-blind, placebo-controlled study, we randomly assigned 32 HSCT recipients with mild/severe BOS to receive budesonide/formoterol or placebo for 6 months. The primary outcome was the change in the FEV1 after 1 month of treatment (M1) compared with the baseline value. Patients were unblinded at M1 if there was no improvement in the FEV1. Those who had initially received placebo were switched to budesonide/formoterol. Intention-to-treat analysis was performed to assess the primary outcome. Additional analyses took scheduled treatment contamination into account. At M1, the median FEV1 increased by 260 ml in the budesonide/formoterol arm compared with 5 ml in the placebo arm (P = 0.012). The median increases in the FEV1 at M1 relative to the baseline value for the treated and placebo groups were 13 and 0%, respectively (P = 0.019). Twenty-five patients received budesonide/formoterol during the study. The median difference in the FEV1 between the baseline and after 1 month of treatment for these patients was +240 ml (P = 0.0001). The effect of budesonide/formoterol on the FEV1 was maintained in the 13 patients who completed 6 months of treatment. Budesonide/formoterol administration led to a significant improvement in the FEV1 in patients with mild/severe BOS after allogeneic HSCT. Clinical trial registered with www.clinicaltrials.gov (NCT00624754).