The medical records of 100 patients who received 113 temporary transvenous pacemakers were reviewed to determine the incidence of complications and malfunction. Malfunction, defined as failure to capture or sense, or both, occurred in 42 (37 percent) of 113 temporary pacemakers. The initial malfunction occurred within 24 hours in 21 (50 percent) and within 48 hours in 36 (86 percent) of the 42 pacemakers. Although the incidence of malfunction was not significantly different for brachial and femoral venous pacing catheters, 7 (37 percent) of 19 brachial venous pacemakers required repositioning or replacement compared with 8 (9 percent) of 91 femoral venous catheters (p = 0.005). Thirty-seven complications occurred in 23 (20 percent) of 113 episodes of pacing; ventricular tachycardia during catheter insertion, fever and phlebitis were the most common complications. No complication resulted in death. The incidence of complications and perforation was greater for brachial than for femoral venous pacemakers (p less than 0.05). Sepsis, local infection and pulmonary embolus occurred only with femoral venous pacemakers. Sepsis, phlebitis and pulmonary embolus were more common with temporary pacemakers in place for 7 hours or longer (p = 0.04). Recognition to the problems peculiar to each pacing catheter site and shortening the duration of pacing should help minimize problems with temporary pacing.