Background: Since 2005, we have been conducting prospective surgical site infection (SSI) surveillance and infection control according to the U.S. Centers for Disease Control and Prevention guidelines for patients who undergo gastrointestinal surgery at our institution. Surgical site infection occurs with greater frequency in emergency than in elective surgery because of factors such as ill-conceived preoperative preparation and dirty operations. A large number of studies have been reported on risk factors for SSI in elective colorectal surgery; however, there are few papers on such factors in emergency colorectal surgery. The aim of this study was to identify risk factors for incisional SSI in emergency colorectal surgery. Methods: Using our SSI surveillance database, we analyzed retrospectively 78 patients who underwent emergency colorectal surgery between 2005 and 2010. Univariable and multivariable analyses were used to identify risk factors for incisional SSI. Moreover, we subclassified dirty-infected operations (class IV) by the extent of contamination according to our own definition to study the incidence of incisional SSI at each degree. Results: The incidence of incisional SSI was 32.1% (25/78 patients). By univariable analysis, seven parameters correlated with a higher risk of incisional SSI: Surgical incision class III-IV, obesity (body mass index 25kg/m(2)), American Society of Anesthesiologists score 3 or 4 points, chronic kidney disease (serum creatinine concentration >1.2mg/dL), blood loss 200mL, blood transfusion, and ventilator support. Multivariable analyses showed that surgical incision class III-IV (odds ratio [OR] 5.9; 95% confidence interval [CI] 1.7, 25.2) and obesity (OR 11.9; 95% CI 2.1, 87.8) were independent risk factors for incisional SSI. The incidence of incisional SSI in colon perforation with generalized contamination was statistically higher than that in prepared colon perforation and colon perforation with localized contamination (82.4% vs. 25.0%; p<0.001). Conclusions: The risk factors for incisional SSI in emergency colorectal surgery were incision contamination and obesity. Moreover, the incidence of incisional SSI among the incision class IV operations increased significantly with increasing extents of contamination. As a tactic for management of dirty abdominal wounds, we suggest that primary skin closure is suitable in cases of perforation of a prepared colon or colon perforation with localized contamination. On the other hand, in cases of colon perforation with generalized contamination, delayed primary skin closure or leaving an incision open to heal by secondary intention should be considered.