Pulmonary function tests were performed in 15 thalassemic patients (age 5 years 8 months to 18 years 6 months), receiving both regular transfusions and desferrioxamine, to determine the presence and nature of any abnormalities in lung function. Reactive oxidant production from neutrophils was measured simultaneously to ascertain if a causal relationship existed between free radical production and tissue damage in the lungs. Mean total lung capacity, mean residual volume, and mean forced vital capacity were significantly reduced, indicating a restrictive pattern of lung function abnormality. In addition, the carbon monoxide diffusion was low, and hypoxemia was present in 6 of 13 patients tested. These pulmonary function abnormalities did not correlate with age, cumulative volume of transfusion, or serum ferritin levels. In addition, neutrophil reactive oxidant status did not correlate with these or with pulmonary function parameters. These results indicate that neutrophil-derived oxygen free radicals do not appear to be a major cause of lung function abnormalities in thalassemics.