SUMMARY Thirty patients with right ventricular (RV) and 15 with coronary sinus (CS) pacing catheters were studied by M-mode echocardiography. RV catheters, detected in 23, appeared as linear echoes in the right ventricle during mitral valve recordings in 12, adjacent or superimposed on the tricuspid valve (TV) in 14, and immediately anterior to aortic root and pulmonary valve echoes in two with a redundant loop in RV outflow. In three with complete heart block, prominent systolic anterior movements of the TV occurred when atrial systole coincided with ventricular systole, probably due to catheter-induced TV "buckling" or exaggerated TV annular motion. Catheter echoes mimicked TV recordings in three, since its motion pattern was similar, although delayed and mimicked prolapsing right atrial myxomas in two because of multilayered complexes behind TV, while reverberations cluttering the left ventricle simulated structural echoes present in that cavity. CS catheters, detected in 14 as linear echoes in the area of atrial septum recorded behind the TV, showed typical small humps in late diastole/early systole. Cross-sectional echocardiography with a mechanical sector scanner demonstrated RV catheters at the RV apex in five of seven patients, while CS catheters were detected near the base of the atrial septum in three of five patients. Echocardiography has the potential to localize pacing catheters which are occasionally difficult radiologically or electrocardiographically. Failure to recognize catheter echo patterns may result in errors in echocardiographic interpretation.