Indwelling cardiac catheters by producing local mechanical stimulation or trauma can induce electrocardiographic (ECG) patterns which simulate known electrophysiologic phenomena. Catheter-induced ECG patterns were analyzed in 447 consecutive patients undergoing electrophysiologic studies. Iatrogenic nature of these patterns was suggested by absence prior to placement of catheter; sudden appearance with catheter placement and disappearance with catheter repositioning; reoccurrence with remanipulation of catheters; and simulation (in some cases) by programmed electrical stimulation from the catheter. Common catheter-induced patterns were: right bundle branch block (RBBB) lasting less than 24 hours occurred in 19 patients; transient third degree atrioventricular block in His-Purkinje system developed in 3/13 patients with pre-existing left BBB; catheter-induced ventricular pre-excitation which simulated ECG patterns of type B Wolff-Parkinson-White syndrome; fortuitous synchronization of right ventricular excitation from the catheter, and left ventricular excitation from sinus beat resulted in normalization of the QRS complexes in 5/68 patients with pre-existing RBBB; premature beats from the atria, right ventricle, and His bundle, which were common, resulted in complex ECG patterns. These iatrogenic ECG patterns must be identified in order to avoid errors in interpretation.