Intracardiac surgery for rheumatic and congenital heart disease entails direct trauma to both normal and abnormal endocardium. This communication inquires into the incidence and nature of the endocardial infections that develop subsequent to this injury. On the basis of an examination of 2,263 patients operated upon for acquired and congenital heart disease during a 5-year period terminating in November 1955, bacterial endocarditis appears to be an infrequent complication of surgery, is caused by organisms not so commonly encountered in unoperated patients, and is characterized by a clinical pattern quite different from that ordinarily associated with bacterial endocarditis. The rate of attrition in this group of patients is high, and unquestionably is related to the antibiotic resistance of the unusual organisms and the severity of the basic heart disease.