Our aim was to determine if BMT induces AHR as assessed by methacholine challenge test (MCT) and to determine a possible correlation between AHR and pulmonary complications. The study was Prospective. Subjects: consecutive patients were referred for pulmonary function tests (PFT's); MCT and clincical evaluation before SCT and were followed after transplantation for at least a year. The study was approved by the ethics committee and parental consent was obtained. Clnical evaluation included medical history: type of disease, previous chemo/radiotherapy. Type of SCT was recorded (allogeneic/ autologous). respiratory evaluation included: Respiratory history; previous asthma medications; allergy/ parental smoking; Physical exam; CXR O2-Sat. on rest and exercise. PFT's included: Spirometry, lung volumes, diffusion capacity; MCT prior and 2-6 months post SCT; Follow-up: 6-12 months post SCT. Pulmonary complication: Any respiratory symptoms or signs: cough, SOB, hemoptysis, hypoxemia; New chest radiological finding; Number of hospitalizations due to pulmonary complications.