Transesophageal echocardiography was used to evaluate pulmonary venous flow velocity and pulmonary venous diameter of both the transplanted and native lungs in six single lung transplant recipients. Mean pulmonary venous velocity (50 +/- 10 versus 27 +/- 8 cm/sec) and pulmonary venous diameter (1.39 +/- 0.16 versus 0.98 +/- 0.18 cm) were significantly greater in the transplanted lung than in the native contralateral lung. An index of allograft perfusion, QD-transesophageal echocardiography (pulmonary venous velocity x pulmonary venous diameter), correlated highly with previously measured technetium 99m-labeled macroaggregated albumin quantitative lung perfusion studies (r = 0.94). A pressure gradient in pulmonary venous flow velocity across the left atrial anastomosis was detected in two patients (8 and 12 mm Hg). Analysis of previous resting supine and upright incremental hemodynamic exercise testing showed no significant differences in these two patients with respect to maximum oxygen uptake, mean pulmonary arterial pressure, pulmonary capillary wedge pressure, cardiac index, or pressure-flow relationships. Therefore these left atrial anastomotic gradients did not appear to adversely affect the pulmonary vascular response to incremental exercise. Transesophageal echocardiography may be an invaluable technique in the expedient evaluation of cardiac function and allograft perfusion after lung transplantation.