This review outlines the evolution of risk rates of surgical site infection adopted by the Study on the Efficacy of Nosocomial Infection Control (SENIC) and the National Nosocomial Infections Surveillance (NNIS) system, and evaluates their advantages and limitations. The types of surgical procedures for which the NNIS index has proven to be an efficient system for the classification of patients are presented. The necessity of adapting the NNIS index to take into account specific characteristics associated with hospitals and practices in various regions and countries is stressed. The requirement for further testing and development of the rate concerning specific procedures, particularly those involving orthopaedic surgery, is discussed.