The risk of life-threatening ventricular arrhythmias complicating coronary angiography is reportedly increased in patients in whom temporary righ ventricular pacemakers are used. Placement of the temporary pacing electrode in the right atrium or vena cava during angiography theoretically removes from the vulnerable ventricle a source of mechanically or electrically induced ventricular arrhythmias. This hypothesis was evaluated in 7,648 consecutive patients who underwent cardiac catheterization with selective coronary angiography, including 103 with life-threatening ventricular arrhythmic complications. The prevalence of ventricular arrhythmias was 6.4 times greater in patients with temporary pacemakers than in those without (7% vs 1.1%, respectively, p less than 0.001). Analysis of 369 cardiac catheterizations concurrently using temporary pacemakers revealed a greater than 4-fold decrease in the frequency of ventricular arrhythmias when the pacing electrodes were located in the right atrium or vena cava rather than the right ventricle (2% vs 9%, respectively, p less than 0.01). No significant difference in indications for temporary pacing could be distinguished between the groups based on location of the pacing electrode or occurrence of ventricular arrhythmias. These findings confirm that there is an increased risk of ionic contrast agent-induced ventricular arrhythmias in patients with chronic underlying conduction disturbances undergoing coronary angiography with concurrent right ventricular temporary pacemakers. This risk can be significantly decreased by placing the pacing electrode in the right atrium or vena cava.