Inclusion Criteria: - New diagnosis of BOS after HCT within 3 months of study enrollment - The baseline absolute FEV1 must be >= 10% lower than the pre-transplant FEV1 as defined by the pre-transplant FEV1 minus the baseline FEV1 - Life expectancy > 6 months at the time of enrollment as judged by the enrolling investigator - All females of childbearing potential must have a negative serum or urine pregnancy test =< 7 days before study drug administration - Subject (or parent/guardian) has the ability to understand and willingness to sign a written consent document Exclusion Criteria: - Patients with recurrent malignancy or disease progression requiring anticancer therapy - Known history of allergy to montelukast, zafirlukast, azithromycin, erythromycin, or clarithromycin - Pregnant females or nursing mothers - Transaminases > 5 X upper limit of normal (ULN) - Total bilirubin > 3 X ULN - Patients currently on daily inhaled steroid (with any inhaled steroid equivalent to fluticasone) for > 1 month preceding enrollment - Patients currently on montelukast for > 1 month preceding enrollment - Patients currently on chronic dosing of azithromycin (>= 3 x/week) for > 1 month preceding enrollment; patients that have taken a Z-pak or other macrolides to treat infection are eligible - Patients requiring > 1.2 mg/kg/day of prednisone (or equivalent steroid) - Receipt of any non-Food and Drug Administration (FDA) approved study medication within the last 4 weeks (this does not apply to use of FDA-approved drugs for an off-label indication) - Evidence of any viral, bacterial or fungal infection not responding to appropriate treatment - Patients with clinical asthma (variable and recurring symptoms of airflow obstruction and bronchial hyper-responsiveness) - Patient with any condition that, in the opinion of the investigator, would interfere with the subject's ability to comply with the study requirements - Patients on long-term oxygen therapy - Patients with uncontrolled substance abuse or psychiatric disorder - Patients who are deemed (by the local principal investigator [PI] or the pulmonary function test [PFT] lab) unable to reliably perform pulmonary function tests