Objective: To identify the risk factors for infections in surgical sites, calculate length of procedures and validate the risk score National Nosocomial Infections Surveillance System (NNISS). Material and Methods: A cross-sectional study was conducted in a third level institution in 2009-2010 in 486 surgeries. Infections were classified according to criteria of the Centers for Disease Control and prevention: primary superficial incisional, deep incisional and organ/space. NNISS risk was classified by adding a point to ASA 3 or higher and/or contaminated or dirty wound and/or the time t above the 75 percentile. Results: The time t for the whole surgery was 107 minutes, the infection frequency was 10.9%. Infection were associated with anemia (OR: 3.0), diabetes (OR: 2.8), smoking (OR: 2.6), the stay prior to surgery (OR: 2.6) and single dose of prophylactic antibiotics (OR: 1.03), the index NNISS 3 (OR: 3.6), contamination of the surgical wound (OR: 2.0) and ASA(OR: 2.3). Conclusions: the NNISS index has been validated; underlying problems must be controlled before elective surgery and should be reduced pre-surgical stay.