The infection of an endocardial prosthesis, either valvular or septal patch, is a frequent and dangerous post-operative complication. Eleven patients having postoperative endocarditis (9 on valve prosthesis and 2 on ventricular septal patch) are the subject of this paper. Six of the 7 infected "early" had Gram-negative bacteria in the blood cultures, while staphylococus aureus, streptococcus viridans and klebsiella pneumoniae were responsible for 3 of the 4 "late" infections. In 2 patients, one "early" and one "late" the causative bacteria were not identified. The definite prevalence of Gram-negative flora in post-operative endocarditis may have been facilitated by the routine use of antibiotics after open-heart surgery. Three of the 11 patients survived, one after replacement of the infected prosthesis and two after prolonged specific antibiotic treatment. On the basic of this experience, compared with other written reports, the authors propose to treat postoperative endocarditis medically for approximately 5 weeks, with full doses of specific antibiotics, reserving for surgical treatment only the cases of prosthetic malfunction, left atrial thrombosis, peripheral embolization and in those patients where the medical treatment fails.