Aims: Correct sizing of a left atrial appendage (LAA) closure system is important to avoid redeployment of the device and peri-device leaks. The aims of this study were to assess the significance of two-dimensional transoesophageal echocardiography (2D-TEE), real-time 3D transoesophageal echocardiography (RT 3D-TEE) and angiography for measuring the size of the LAA landing zone and to determine the impact on sizing an LAA closure device. Furthermore, we investigated the relevance of volume loading on LAA size. Methods and results: In a prospective study, 46 patients underwent 2D-TEE and RT 3D-TEE 24 hours prior to LAA closure, at the beginning of the procedure and just before the procedure after volume loading with an average of 1,035 +/- 246 ml. Angiography was performed immediately before the implantation. Maximal diameter (2.2 +/- 0.4 versus 2.3 +/- 0.4 cm; p<0.01), perimeter (6.5 +/- 1.0 versus 6.8 +/- 1.0 cm, p<0.01) and area (3.2 +/- 1.0 versus 3.5 +/- 1.1 cm(2), p<0.01) of the LAA increased significantly after volume loading. The highest correlation (R) between measurements and LAA device size was found for RT 3D-TEE-derived perimeter (R=0.97) and area (R=0.96), whereas the maximal diameter (R=0.78) measured by 2D-TEE and angiography (R=0.76) correlated less closely. Sizing based on an RT 3D-TEE-measured perimeter resulted only in 4% of undersizing the implanted device. Peri-device leaks occurred in seven cases (15%) and were associated with a lower compression of LAA devices (7 +/- 1.3% versus 14 +/- 3.2% for patients without leaks, p<0.001). Conclusions: Volume loading before LAA closure increases LAA dimensions significantly. RT 3D-TEE measurements show a closer correlation to LAA closure device size than 2D-TEE or angiographic measurements.