Infective endocarditis is an uncommon but serious disease, with a high morbidity and mortality. Because of its seriousness, and because of the high prevalence of some of its presumed risk factors, e.g., mitral valve prolapse and dental procedures, considerable effort and resources are expended on antibiotic prophylaxis intended to prevent its occurrence. In order to determine the validity of the assumptions about risk factors which underly the recommendations for these interventions, a case-control study is planned in the Delaware Valley. 300 hospitalized incident cases of endocarditis will be identified and matched by age and sex to a community control group. This comparison will be used to investigate host risk factors for endocarditis, especially mitral value prolapse. Then, for those cases with one of the more common cardiac lesions as host risk factors, a second control subject will be selected, matched for age, sex, and that particular cardiac lesion. This second comparison is expected to involve 150 of the cases and 150 new controls. This comparison will be used to determine whether certain antecedent procedures which might increase the risk of bacteremia, especially dental procedures, are in fact risk factors for developing endocarditis. All subjects will undergo a structured interview to obtain information on both host factors and procedures which could represent risk factors for endocarditis, and their medical and dental records will be reviewed for additional information and for validation of the interview data. Univariate analyses, including calculation of odds ratios and confidence intervals, will be followed by stratification and multiple logistic regression, enabling us to evaluate the relative importance of each variable as a risk factor. With this approach, we should be able to shed additional light on risk factors for this important and relatively unstudied disease. The current list of indications for antibiotic prophylaxis can then be refined, eliminating indications that are not true risk factors and adding others that are. Resources now being spent on prophylaxis can then be better focused.