The incidence of ventricular arrhythmias in critically-ill patients during bedside right-sided heart catheterization with a flow-directed balloon-tipped catheter was determined. Twenty-nine of 60 catheterizations (48 percent) were associated with premature ventricular contractions and 20 (33 percent) were associated with ventricular tachycardia. Two patients required antiarrhythmic therapy or a precordial thump to convert ventricular tachycardia. One patient developed ventricular tachycardia and fibrillation and died. Serious catheter-induced arrhythmias, including sustained ventricular tachycardia, may occur during Swan-Ganz catheterization of the critically ill.