Twenty-four of a series of 540 patients undergoing operative treatment of congenital heart disease developed septicemia. In the 20 patients under 6 months of age, mechanical ventilation was used postoperatively; the signs and symptoms of sepsis were vague. One infant in this group is alive. In the four between the ages of 2 and 32 years, one was successfully treated by antibiotics, two required reoperation to replace prosthetic patches with autogenous pericardium, and one died from septic shock. Gram-negative bacteria, resistant to the "prophylactic" antibiotics used, were most commonly encountered. The source of sepsis was the lungs, the operative wound, or an intravenous catheter. In five cases, the source was not apparent. The study suggests that sepsis is a more common cause of death following cardiovascular operations in infants than had been realized. Infected intracardiac prostheses cannot be sterilized by antibiotics and must be removed.