Background The left atrial appendage (LAA) is an important source of thrombus formation.We investigated the feasibility of the recently developed real-time three-dimensional transesophageal echocardiography (RT3D-TEE) method in assessment of the morphology and function of the LAA.Methods Ninety-six consecutive patients (58 males with a mean age of (43.4±12.5) years) who were referred for 2-dimensional (2D) transesophageal echocardiography (TEE) underwent additional RT3D-TEE.LAA morphology was visualized in multiple views.Orifice size,depth,volumes and ejection fraction (EF) of the LAA,were measured.Results All the patients underwent RT3D-TEE examination without complications.Ninety-two patients (95.8％) had adequate images for visualization and quantitative analysis of the LAA.The LAA exhibited great variability with respect to relative dimensions and morphology.LAA orifice area was (3.8±1.2) cm2 with a diameter of (2.4±0.9) cm x (1.4±0.6) cm.The mean depth of the LAA was (2.9±0.7) cm.End-diastolic volume (EDV-LAA),end-systolic volume (ESV-LAA) and EF of the LAA were (6.2±3.7) ml,(4.1±2.8) ml,and 0.35±0.16,respectively.EDV-LAA,ESV-LAA and the orifice area of the LAA in patients with atrial fibrillation (AF) were larger than those without AF,whereas the EF was smaller in the AF patients.Conclusions Defining LAA morphology and quantitative analysis of the size and function of the LAA with superior quality and resolution of images using RT3D-TEE is feasible.This technique may be an ideal tool for guidance of the LAA occlusion procedure.Determination of LAA volumes and volume-derived EF by RT3D-TEE provides new insights into the analysis of LAA function.