Pulmonary diffusing properties and lung volumes were investigated in 44 patients with atrial septal defect, and in 30 of them preoperative and postoperative data were obtained. The patients were divided into three groups according to mean pulmonary artery pressure: less than or equal to 15 mm Hg (group 1), 16-29 mm Hg (group 2), and greater than or equal to 30 mm Hg (group 3). Patients in groups 1 and 2 had a high carbon monoxide transfer test which became normal after surgical correction of their septal defect. In group 3, the carbon monoxide transfer test was normal both before and after operation. As mean pulmonary artery pressure increased there was a progressive reduction in both forced expiratory volume in one second and vital capacity. Patients in group 3 had a low forced expiratory volume in one second, a low vital capacity, and a reduced forced expiratory volume in one second:vital capacity ratio. These abnormalities were not corrected by surgical closure of the septal defect. Formulas were derived from the lung function data, to predict the mean pulmonary artery pressure and the pulmonary:systemic flow ratio. The values predicted when these two formulas were applied to data obtained for patients in this study correlated well with measured values.