We studied the pacemaker lead that had been removed from a patient who suffered three sequential episodes of Staphylococcus aureus bacteremia. This organism was recovered from the surface of the lead. Scanning electron microscopy showed differential colonization of the pacemaker lead. The metal tip, the inner surface and the internal wires were covered with a heavy biofilm of bacteria. The outer silastic surface had no biofilm adherent to it; instead, well-spaced bacterial cells were seen. These observations illustrate why infection of implantable devices persists despite intensive antibiotic chemotherapy.