Background: The lung is one of the organs that can be damaged in patients with thalassemia major (TM). In this study, we aimed to evaluate the pattern of lung impairment using various pulmonary tests and the effects of transfusion on pulmonary functions in patients with TM. Materials and Methods: The study was performed on patients 9-21 years of age prospectively. Spirometric tests, plethysmography, and CO diffusion tests were performed before and 2 h after erythrocyte transfusion. Patients also underwent a methacholine stimulation test 2 h after transfusion. Results: A total of 37 patients with TM were included in the study. There was no change in pretransfusion and post-transfusion spirometric test results. Plethysmographic tests revealed that both the median total lung capacity and the mean vital capacity values have decreased after transfusion. Medians of pretransfusion DLCO values significantly increased after transfusion. There was an impairment in diffusion in 16 (44.4%) patients, whereas it improved in 9 patients after transfusion. Bronchial hyperreactivity was detected in 10 out of 27 patients (37%) via methacholine stimulation test. There was obstructive airway disease in 20 patients (54%) out of 37 patients who had been tested. Conclusions: Patients with TM may have respiratory function abnormalities even while being asymptomatic. An obstructive airway pattern was the most common abnormality in our patient group. We revealed for the first time in the literature the presence and frequency of bronchial hypersensitivity in these patients using the methacholine stimulation test.