Chronic rejection (or Bronchiolitis Obliterans syndrome-BOS) is a major cause of mortality and morbidity after lung transplantation. Because montelukast has been shown to be of some efficacy in a similar disease (Obliterative Bronchiolitis after bone marrow transplantation), the investigators would like to test if montelukast can indeed slow down the progression of chronic rejection after lung transplantation.

• Prospective, interventional, randomized, double-blind, placebo-controlled trial.
• Clinical setting (tertiary University Hospital).
• Investigator-driven, no pharmaceutical sponsor.
• Lung transplant recipients.
• Add-on of study-drug (placebo or montelukast) to 'standard of care'
• 1:1 inclusion ratio (placebo:montelukast).
• Randomisation at diagnosis of chronic rejection after informed consent.

Inclusion Criteria:

• Diagnosis of fBOS
• Signed informed consent
• Age at least 18 years old at moment of transplantation
• Able to take oral medication

Exclusion Criteria:

• Retransplantation
• Previous organ transplantation
• Multi organ transplantation