Intracardiac echocardiography using a new 9-MHz ultrasound catheter was performed in 30 patients undergoing percutaneous catheter mapping and radiofrequency ablation of a tachyarrhythmia, because the imaging capabilities with this intracardiac echocardiographic catheter permit detailed identification of normal and abnormal cardiac anatomy with improved imaging depth. Intracardiac echocardiography is of significant clinical utility during ablation for guiding interatrial septal puncture, assessing placement and contact of mapping/ablation catheters, monitoring ablation lesion morphologic changes, and diagnosing procedure-related complications.