790 The group of potential risk factors for AL that is studied in literature is generally quite similar and contains, in addition to the aforementioned factors, mechanical bowel preparation (MBP) [10], prophylactic drainage (PD) [11], ASA-score [4], prolonged operating time [4], use of corticosteroids [12], anastomotic configuration [13], hand-sutured vs. stapled anastomosis [14], neoadjuvant radiotherapy [15], laparoscopic vs. open surgery [16], and gender [8]. Despite the vast body of evidence on these potential risk factors AL remains poorly understood. Therefore, risk factor analysis should not be limited to these factors. Since ischemia is considered to be one of the causes of AL, studying vascular disease, i.e., atherosclerosis, could be a new approach as suggested by Foster et al. [17]. Analysis of known risk factors for atherosclerosis like hypertension [18], dyslipidemia [19], smoking [20], diabe- tes mellitus [21] could be an interesting addition to the group of potential risk factors. More and more publications show the danger of after- hours medical activity. Gray et al. showed after-hours surgery to be related to higher complications rate [22]. Fechner et al. showed that night time kidney transplantation enhances the risk for complications and graft failure [23]. In addition, several studies have shown that proficiency and situational awareness of physicians is less at night [24–27]. On the matter of colorectal surgery, the time of day at which the patient is operated has never been related to AL. In short, risk factors for AL vary between different populations and, since it is a multifactorial problem, there are probably several unknown risk factors. This means that more studies on this matter, from different hospitals throughout the world should be per- formed, published and compared in order to improve understanding of this complication. This study aims to describe the population of patients with primary colorectal anastomosis treated in a Dutch tertiary referral center and to determine the incidence and risk factors of AL in this population. Materials and methods All patients who have undergone surgery on the colon or rectum in the period 1997–2007 were selected in the electronic archive by means of operation code of the relevant operations. In this population, patients whom received a primary anastomosis, involving colon and or rectum, were selected. The electronic- and paper patient records were used to score patient-, surgery-, and disease-related factors for risk factor analysis and description of the patient population. The patient-related factors analyzed in this study were body mass index (BMI), age, gender, diabetes mellitus (DM), smoking, cardiac- and pulmonary comorbidity, Int J Colorectal Dis (2009) 24:789–795 history of vascular disease, ASA-score, use of steroids, statins, and anti-hypertensive medication (Table 1). The surgery-related factors analyzed in this study were prophylactic drainage (PD), time of surgery, type of operation, anastomotic configuration, blood transfusion, surgeon vs. assistant, mechanical bowel preparation (MBP), laparoscopic vs. open operation, duration of surgery, stapled vs. hand-sutured anastomosis, urgent vs. elective operation (Tables 1 and 2). Operations were considered urgent when it was indicated in anaesthesiolo- gists’ records and the patient’s file. Examples of included surgical emergencies were mechanical obstruction due to colorectal cancer and perforated diverticulitis. The disease-related factors analyzed in this study were the type of neoadjuvant therapy, radio- or chemotherapy or a combination, surgical indication (Table 1). In addition, several outcome parameters like wound infection, pneumonia, urinary tract infection, abdominal wound dehiscence, hospital stay and stay on the intensive care unit were analyzed as well The primary outcome measure was clinically manifest anastomotic leakage, confirmed by imaging or relaparotomy. In case of imaging, leakage was considered to be present when free air or contrast was visible around the anastomosis. In case of relaparotomy, leakage was considered to be present when a dehiscent anastomosis was visualized. Results are summarized as means and standard deviations or medians and ranges for the continuous variables; the categorical variables are summarized in frequencies. Median and mean values were compared between groups with and without AL by means of the Mann–Whitney test or Chi-square test in univariate analyses. Multivariate analysis was per- formed with factors that were significant in univariate analysis and consisted of multiple logistic regression with backwards elimination. In the final model, multiple logistic regression with backwards elimination with all factors, significant or non- significant in univariate analysis, was performed. Operation duration was transformed logarithmically in this analysis to reduce the influence of outlying observa- tions. A p value of 0.05 (two-sided) was considered the limit of significance. Analyses were performed with “SPSS 15.0” statistical software. Results Patient population A total of 739 patients received a primary colorectal anastomosis over the selected period. This number included 90 ileocecal resections, 27 transverse colon resections, 250 right hemicolectomies, 30 subtotal colectomies, 64 left hemicolectomies, 155 sigmoid resections, 18 rectosigmoid