The cost of an antibiotic is easily evaluated in the case of antimicrobial prophylaxis. The other expenses, either direct or indirect costs, are much more difficult to assess. Studies evaluating the economical impact of the prophylactic antibiotics are rather rare. Antibiotic prescription for prophylaxis is evaluated to represent about 20 to 30% of the total antibiotic administration in French hospitals. The most significant studies evaluate the cost/benefit ratio and demonstrate the advantage of prophylaxis in orthopaedic, vascular and gynaecologic surgery.