Corynebacterium jeikeium is the name recently given to a group of coryneform rods, formerly known as the JK group, and associated with a wide range of serious infections. We describe five patients with C. jeikeium endocarditis observed in two Belgian hospitals over a period of 5 years. In three cases, the infection occurred on prosthetic heart valves between 1.5 and 5 months after surgery. The two other patients, with known mitral insufficiency, presented with endocarditis on native valves. In four patients, bacteriologic and clinical cure followed treatment with vancomycin. The fifth patient died despite eradication of the coryneforms by combination antibiotic therapy. We know of no previous reports of C. jeikeium endocarditis on native heart valves.