Surgical site infection (SSI) is one of the most common nosocomial infections. The incidence of SSIs varies in different studies, also in studies including only gastrointestinal surgery, and there has not yet been found an estimate to the actual incidence of SSIs. There are many risk factors associated to SSIs, both host factors and operation-related factors. The introduction of antimicrobial prophylaxis has been one of the most important means to reduce the incidence of SSIs. Infections cause a substantial burden both to the patient and to hospital budgets. In our study we followed patients for 45 days after surgery where development of SSI was monitored. In all operations the gastrointestinal tract was opened. All patients were operated within the year 2010 at The Gastroenterologic Department, Oslo University Hospital, Ullevål. We also registered different host and operation-related factors to see if they occurred differently in those who developed a SSI compared to those who did not. 203 operations were included and we found an incidence of SSIs of 36 %. This is much higher than numbers figuring in other studies. This might partly be explained by the fact that we followed the patients for a longer period of time, included only operations where the gastrointestinal tract was opened and did not exclude emergency operations. Ullevål hospital is a referral hospital and patients might have more complex diseases than patients treated at a general gastroenterological department. There is also a possibility that this is a coincidental finding, and we recommend that a new study will be carried out.