Diaphyseal femoral and tibial fractures are in the spotlight within the traumatology-orthopedics scenario. Intramedullary nailing (IMN) remains the method of choice for treating these fractures, both open and closed ones. Occurrences of surgical site infection (SSI) related to this type of osteosynthesis are a challenge for all the professionals involved in patient healthcare. The reported incidence of SSI after IMN ranges from 0.9 to 17.5%. The majority of the data comes from retrospective studies and as part of case series descriptions, with little detail about the criteria used for defining and searching for cases of infection. Regarding the potential risk factors for this complication, previous use of external fixators, occurrence of open fractures and the severity of exposure according to the Gustilo-Anderson classification were indicated in a few retrospective studies as being possibly related to this complication. The objectives of the present observational cohort study are: 1. To determine the incidence of SSI related to IMN for fixation of diaphyseal femoral and tibial fractures in patients treated in a university traumatology and orthopedics reference hospital in São Paulo, Brazil; 2. To evaluate the risk factors related to the occurrence of this type of infection.