Three patients who had coryneform bacterial endocarditis affecting a prosthetic valve are presented and 64 case reports are reviewed. Diagnostic difficulties occur because coryneform bacteria often are fastidiuos, with long incubation periods, and often contaminate blood cultures. Although some coryneform bacteria are killed by penicillin G, many are resistant to most of the commonly used antibiotics. Vancomycin is bactericidal in resistant strains studied. Treatment with vancomycin is indicated until in vitro bactericidal data are available. Coryneform endocarditis often occurs on prosthetic valves, thus making therapy and its evaluation even more difficult.