To determine whether prophylactic lidocaine could decrease the incidence of advanced ventricular arrhythmias, 62 patients undergoing 67 pulmonary artery catheterizations were given lidocaine or placebo before and during catheterization. Advanced ventricular arrhythmias occurred in 42 of the 67 catheterizations (63 percent). In 18 of 31 patients receiving lidocaine (58 percent) arrhythmias developed, whereas 24 of 36 patients who received placebo (67 percent) had evidence of arrhythmias. These differences were not significant. However, patients with catheterization times of less than 20 minutes who were treated with lidocaine had less ectopy (25 percent) than patients treated with placebo (68 percent) (p less than 0.05). Two patients has sustained ventricular tachycardia and both were receiving placebo. No complications of lidocaine prophylaxis were noted. Prophylactic lidocaine appears to decrease the incidence of mechanically induced arrhythmias in critically ill patients undergoing catheterization that is not prolonged.