Background and Objectives: Duration of antibiotic prophylaxis for cardiac surgery is still debated and controversial. International guidelines are vague: French guidelines recommend an intraoperative administration, while the Society of Thoracic Surgeons' guidelines suggest that optimal postoperative prophylactic antibiotics be given for 48 hours or less. Very few studies have compared the same antibiotic with 2 different administration durations. The study was designed to compare the efficacy of 24-hour administration of cefamandole vs intraoperative cefamandole to prevent deep sternal wound infection and endocarditis after cardiac surgery. Methods: This retrospective and observational study compared the rates of severe surgical site infections (deep sternal wound infection, endocarditis) after cardiac surgery between period 1, 01/01/2008-31/08/2008, with 24-hour administration of cefamandole, and period 2, 01/09/2008-30/04/2009 with intraoperative cefamandole. Results: Among 933 patients, 14 patients (1.5%) developed surgical site infection during the 16-month study: 1.3% during the first period and 1.7% during the second (ns). The populations (470 patients in period 1 and 463 in period 2) were homogeneous and comparable for pre-, intra-and postoperative characteristics. Surgical site infection characteristics (pathogens involved, time to diagnosis) and consequences (longer hospital stay, outcomes) were comparable in the 2 groups. Conclusions: Intraoperative cefamandole was as safe as its 24-hour administration to prevent deep sternal wound infection and endocarditis after adult cardiac surgery.