Anastomotic leakage (AL) is the most important and one of the most serious complications after colorectal resections with primary anastomosis. Any factors that contribute to increase the risk of AL should be identified and - if possible - eliminated. Non-steroidal anti-inflammatory drugs (NSAIDs) are often used for treating pain after surgical procedures, among these also colorectal resections. The objective of this Ph.d. thesis was to investigate whether the use of NSAIDs in the postoperative period increases the risk of AL, and investigate the effect on pathophysiological mechanisms. In order to achieve this, the following studies were performed. Study I was a retrospective, case-control study in 75 patients undergoing laparoscopic colorectal resection for colorectal cancer. 33 of these patients received the NSAID diclofenac in the postoperative period; the remaining 42 did not receive any NSAID. There were significantly more ALs among the patients receiving diclofenac (7/33 vs. 1/42, p=0.018). In uni- and multivariate logistic regression analyses, diclofenac was the only factor associated with increased AL rate. This study functioned as a hypothesis generating study and laid the ground for the subsequent studies. Study II was an experimental, randomized, case-control study in 32 Wistar rats. The rats had a colonic anastomosis performed and were randomized to diclofenac or placebo treatment. After three days, the rats were sacrificed and the anastomoses were harvested. First, the anastomotic strengths were tested by longitudinal; subsequently, the levels of the enzyme cyclooxygenase-2 (COX-2) in the anastomotic tissues were measured. There was no difference among the groups with regard to anastomotic strength, but the animals treated with diclofenac had significantly lower COX-2 levels (median (range) 1.30 (0.42-3.31) ng/mg vs. 2.44 (0.88 - 18.94) ng/mg, p