The study investigates the source of *Staphylococcus aureus* bacteremia, a severe infection with significant morbidity and mortality. The researchers examined *S. aureus* isolates from blood and nasal specimens to determine if the organisms in the bloodstream originated from the patient's own flora. In a multicenter study, 219 patients with *S. aureus* bacteremia were tested, and 723 isolates were collected and genotyped. In a second study, 1640 *S. aureus* isolates from nasal swabs of 1278 patients over five years were compared with isolates from the blood of patients who later developed *S. aureus* bacteremia. Results showed that in 180 out of 219 patients (82.2%), the blood isolates were identical to those from the anterior nares. In the second study, 14 out of 1278 patients who had nasal colonization with *S. aureus* subsequently developed bacteremia, and in 12 of these 14 patients (86%), the isolates from the nares were clonally identical to those from the blood. These findings suggest that a substantial proportion of *S. aureus* bacteremia cases may be of endogenous origin, originating from nasal carriage. The study supports the importance of strategies to prevent systemic *S. aureus* infections by eliminating nasal carriage.The study investigates the source of *Staphylococcus aureus* bacteremia, a severe infection with significant morbidity and mortality. The researchers examined *S. aureus* isolates from blood and nasal specimens to determine if the organisms in the bloodstream originated from the patient's own flora. In a multicenter study, 219 patients with *S. aureus* bacteremia were tested, and 723 isolates were collected and genotyped. In a second study, 1640 *S. aureus* isolates from nasal swabs of 1278 patients over five years were compared with isolates from the blood of patients who later developed *S. aureus* bacteremia. Results showed that in 180 out of 219 patients (82.2%), the blood isolates were identical to those from the anterior nares. In the second study, 14 out of 1278 patients who had nasal colonization with *S. aureus* subsequently developed bacteremia, and in 12 of these 14 patients (86%), the isolates from the nares were clonally identical to those from the blood. These findings suggest that a substantial proportion of *S. aureus* bacteremia cases may be of endogenous origin, originating from nasal carriage. The study supports the importance of strategies to prevent systemic *S. aureus* infections by eliminating nasal carriage.