Background: AcuNav ultrasound catheter (UC) (10F, 5.5-10 MHz) has unique advantages for left heart imaging with its 4-way tip flexible maneuverability, maximal 16-cm intracardiac imaging depth, and Doppler and color flow imaging capability. Methods: We assessed the initial use of this UC in 40 consecutive patients (34 men; age 53 ± 11 years old). All patients were also undergoing transseptal catheterization for percutaneous catheter mapping and ablation of either left atrium (focal initiated atrial arrhythmia/fibrillation, n = 32) or left ventricle (ventricular tachycardia, n = 4), or transcatheter atrial septal defect closure (n = 4) procedures. During each procedure, the UC was placed in the right atrium, superior vena cava, or right ventricular inflow/outflow tract. Results: In all patients, UC successfully guided transseptal catheterization and provided imaging of normal or aberrant anatomy of the right/left atrial (interatrial septum, fossa ovalis, appendages, 4 pulmonary vein ostia) and right/left ventricular (valves and papillary muscles) structures. UC was important in early identification procedure complications, including pericardial effusion (n = 2, detected before systematic hemodynamic deterioration) and thrombus formation on sheaths deployed in the right atrium (n = 9) and left atrium (n = 2, early elimination with management of the sheath). With Doppler and color flow imaging, UC provided effective monitoring of increased flow velocity of all ablated pulmonary vein ostia and detection of patent foramen ovale (n = 6) or residual trivial/small atrial septal defect posttransseptal catheterization (n = 2). UC was also used to successfully image and guide transcatheter closure of atrial septal defect with positioning of the cardioseal septal occluder (Nitinol Medical Technologies Inc, Boston, Mass) and color Doppler imaging of no significant residual shunt. Conclusion: AcuNav UC with Doppler and color flow imaging has significant use, especially during left heart ablation. Uses include guidance of transseptal and mapping/ablation catheters and closure devices, and prompt diagnosis of cardiac complications. (J Am Soc Echocardiogr 2002;15:1301-8.)