Background-The Intracardiac Echocardiography Guided Cardioversion Helps Interventional Procedures study evaluated the concordance of intracardiac echocardiography (ICE) with transesophageal echocardiography (TEE) in patients with atrial fibrillation (AF). Methods and Results-Patients with AF undergoing right heart catheterization underwent left atrium (LA) and interatrial septal (IAS) imaging by TEE and ICE. A blinded comparison of the 2 modalities was performed at a core laboratory. Ninety-five patients aged 58±12 years completed the study. The LA was profiled in all patients with both techniques, and concordance for image quality was 96%. LA appendage (LAA) imaging was achieved in 85% with ICE and 96% with TEE. There was no difference in the presence of spontaneous echo contrast between ICE and TEE during LA imaging, but there was a trend toward a greater incidence in the LAA with TEE (P=0.109). Intracardiac thrombus was uncommonly seen (TEE, 6.9%; ICE, 5.2%). The concordance for the presence or absence of thrombus was 97% in the LA and 92% in the LAA, but the latter was detected more frequently with TEE. IAS imaging was achieved in 91% with ICE and in 97% with TEE (P=0.177). Concordance for patent foramen ovale and atrial septal aneurysms was 100% and 96%, respectively. A negative ICE examination was associated with absence of dense echo contrast or thrombus on TEE in 86%. Conclusions-This study provides validation for the use of ICE for LA and IAS imaging. ICE imaging was less sensitive compared to TEE for LAA thrombus identification. © 2010 American Heart Association, Inc.