A doubJe blind study of antibiotic prophyllaxls in fractures of rhe neck uf rhe íernur in elderly patients, was made for the following reasons: - the high incidence in this hospital. - the great riske of infection in elderly patients with their associatcd pruhlerns IIf general health, nutrition... - slnce it is an example of replacement surgery, these results may be applied ro total arthroplasty of the hipo After study of the literature, and consideration of the pharmacokinetics IIf the cephalosporins most frequently used, together with the epidemiology IIf the Trauma and Orthopaedics ward of this Hospital (with computerized antibiograms), «cefaznlin» was chosen for prophyllaxis. The treatement was as follows: 1 gram intravenously 15 minutes preoperatively. 1 gram intravenously every 6 hours for the following 3 days. The incidence of infection in the control group which received no antibiotic treatement was of 12'5 %, this was reduced to 3'S % in the group receiving antibiotic prophyllaxis. It should be pointed out that all patients received the same type of prothesis and the same therapeutic route for the antibiotic. These cases were all treated by the same surgical team.