Pulmonary function tests were performed at rest and following blood transfusion on 17 patients with thalassemia major. At rest, diffusion capacity was significantly reduced (mean of 57 percent), and forced vital capacity was mildly reduced (mean of 85 percent). In two of the patients, arterial oxygen tension was low. Other pulmonary function results were in the normal range. Following blood transfusion, a significant drop was seen in arterial oxygen tension (PaO2 of 22 mm Hg) and in forced vital capacity (FVC) of 32 percent. These changes were dramatic and require further explanation.