Single-lung transplantation (SLT) is a viable option for patients with end-stage pulmonary disease. After successful SLT, pulmonary blood flow is preferentially shifted to the transplanted lung, creating a flow differential. Lack of flow differential may be indicative of potential vascular complications such as anastomotic stenosis or thrombosis. To assess the ability of transesophageal echocardiography (TEE) in estimating lung flow differential in patients undergoing SLT, biplane TEE was prospectively performed in 18 consecutive patients undergoing SLT early (24 to 72 hours), and in 10 of them late (3 to 6 months) after surgery. Right and left pulmonary vein flow were calculated as Qν=A · VTI, where A, the pulmonary vein area, was derived as π ·(D/2)² and VTI is the velocity time integral of the pulmonary vein spectral display. Lung flow differential was calculated as the ratio of right (RQν) or left (LQν) pulmonary vein flow to total pulmonary venous flow (RQν + LQν). Lung perfusion imaging scintigraphy (technetium-99m) was used for comparison. Pulmonary vein velocity time integral of transplanted lung was significantly greater than that of native lung (34 ± 9 vs 18 ± 8 cm, p <0.001). Percent differential lung flow derived by perfusion imaging scintigraphy and by TEE showed a good correlation (r = 0.67, p <0.001). Pulmonary artery anastomoses were seen in all 12 right-lung recipients, and in 4 of the 6 left-lung recipients; no significant stenosis was noted in the arteries visualized. The pulmonary venous anastomoses were imaged in all patients. Small, nonocclusive pulmonary vein thrombi were seen in 1 patient. In conclusion, TEE is a useful method for calculating lung flow differential in patients undergoing SLT. In addition, TEE provides superb direct visualization of the venous and arterial anastomoses in most patients. Contrary to previous reports, the overall incidence of anastomotic complications is relatively low.