To evaluate the long-term effect of systemic antibiotic prophylaxis at pacemaker surgery, an analysis was made of infections in 272 patients submitted to 303 consecutive pacemaker operations. The follow-up time was 24-33 months. Antibiotic prophylaxis had been given in all cases. The overall infection rate was 4.0%: The incidence after new implant was 3.8%, generator replacement 2.4%, electrode replacement 15% and early reoperation 4.2%, with the only statistically significant difference between electrode replacement and generator replacement. The interval between operation and appearance of infection was 3 days to 18 months. The causal microorganisms were methicillin-resistant Staphylococcus epidermidis in four patients, methicillin-sensitive S. epidermis in one patient and methicillin-sensitive S. aureus in three. An anaerobic gram-positive coccus was cultured in one patient and a nonenteric gram-negative rod in another. Five infections were cured by antibiotic treatment alone, but in seven cases surgery was also needed. The incidence of infection was significantly increased when potential predisposing factors--diabetes mellitus, postoperative hematoma and use of a temporary electrode--were present.