Anterior resection for rectal cancer carries the risk of serious complications, especially fistulas at the site of anastomosis. Numerous factors have been shown to impact anastomotic leakage. The results of studies on the influence of obesity on the frequency of anastomotic leakage after rectal resection performed due to cancer have been contradictory.The aim of the study was to evaluate the relationship between body mass index (BMI) and frequency of anastomotic leakage after anterior rectal resection performed due to cancer. Material and methods. This retrospective analysis included 222 subsequent patients who had undergone anterior resection due to cancer with an anastomosis formed with a mechanical suture. The patients were divided into 3 groups depending on their BMI quartile as follows: Group I, BMI < 23.8 kg/mResults. Anastomotic leakage occurred in 8 (3.6%) patients. Fistulas occurred in 4 out of 61 patients (6.56%) in group I, which was the highest incidence of fistulas for all 3 groups. In group II, fistulas occurred in 2 out of 55 patients (3.63%), and similarly, in group III, they occurred in 2 out of 106 patients (1.87%). The differences found in the frequency of fistulas between groups were not statistically significant (p=0.31). The logistic regression analysis did not show any relationship between leakage and age (p = 0.55; OR = 1.02; 95% CI: 0.95 - 1.1), sex (p = 0.97; OR = 0.97; 95% CI: 0.22 - 4.25) or BMI (p = 0.27; OR = 0.58; 95% CI: 0.22 - 1.53).Conclusions. The results of our study show that BMI did not have any influence on the frequency of anastomotic leakage after anterior rectal resection performed due to cancer.