Intracardiac echocardiographic imaging (ICE) is increasingly useful in guiding complex electrophysiological procedure. We assessed the utility of ICE in 25 consecutive pts using a new 9 MHz ultrasound catheter. All pts (12 male, age 38 +/- 16 years) were also undergoing percutaneous catheter mapping and ablation of a tachyarrhythmia (accessory pathway, n = 4; sinus node, n = 11; atrial tachycardia, n = 2; type I atrial flutter, n = 4; ventricular tachycardia, n = 2). Structural heart disease was present in 8 pts. During each procedure, the imaging catheter was placed in the right atrium (RA), superior vena cava, right ventricular inflow and outflow tract.Results: In all pts, ICE identified distict endocardial structures with excellent detail, including crista terminalis, RA appendage, fossa ovalis, caval orifices, right pulmonary vein orifices and all cardiac valves. ICE was important in identifying aberrant anatomy (Ebstein's anomaly, interventricular septal defect, atrial septal defect and lipomatous hypertrophy, torn chordae tendinae) or in detecting procedure complications (pericardial effusion, intraatrial thrombus, atrial perforation). In some pts, ICE was the primary ablation catheter guidance technique (e.g. sinus node modification). In all pts, ICE was contributory to the mapping and ablation process by guiding mapping catheters to anatomically distinct sites and/or assessing stability of the electrode-endocardial contact. ICE was also used to guide atrial septal puncture (n = 3) or RA basket catheter placement (n = 4).Conclusions: 9 MHz ICE has significant utility during catheter ablation, including guidance of mapping/ablation catheters, identifying complex anatomy, and prompt diagnosis of cardiac complications.