Background: Compared to laparoscopic bariatric surgery, open Roux-en-Y gastric bypass surgery (RYGBP) carries an increased risk for postoperative incisional hernia formation. Recently, there has been increasing interest in applying adjunctive techniques for abdominal wall closure in order to reduce the incidence of incisional hernias after open bariatric surgery. The purpose of this study was to determine the incidence and risk factors for incisional hernia after open RYGBP. Methods: A retrospective analysis of a prospectively collected outcomes database was performed on all patients undergoing open RYGBP at a single institution by a single surgeon using a standardized surgical technique between 1999 and 2004. Both univariate and multivariate models were used to determine the factors influencing the development of a postoperative incisional hernia within one year after open RYGBP. Results: During this 5-year period, 588 patients underwent open RYGBP for morbid obesity. All patients were followed postoperatively by the primary surgeon at 3-month intervals. The mean age of all patients was 42 years, and the mean BMI was 55 kg/m2. Within one year of the time of surgery, incisional hernias were detected clinically or radiographically in 133 patients (22.6%). Age, BMI, and preoperative diabetes mellitus did not predict postoperative hernia formation. Superobese patients (BMI >/= 60 kg/m2) were not at increased risk for an incisional hernia. Among patients who developed incisional hernias, 49% had preoperative sleep apnea; whereas, sleep apnea was present in only 30% of patients who did not develop a postoperative hernia, p = <0.0001. Other significant univariate risk factors for incisional hernia included male gender (risk ratio (RR) = 2.28, p = 0.0001) and the development of a postoperative surgical site infection (RR = 3.19, p = 0.0007). African-American patients were protected against incisional hernia formation (RR = 0.56, p = 0.02). No patient developed an acute wound dehiscence or hernia-related bowel incarceration after open RYGBP. Conclusions: Incisional hernias occur in nearly a quarter of patients who undergo open RYGBP. Preoperative independent predictors of incisional hernia formation include sleep apnea and male gender. Patients at high risk for postoperative incisional hernia after open RYGBP may benefit from techniques aimed at supporting abdominal fascial closure at the time of surgery.