OBJECTIVE to evaluate the effect of perioperative administration of symbiotics on the incidence of surgical wound infection in patients undergoing surgery for colorectal cancer. METHODS We conducted a randomized clinical trial with colorectal cancer patients undergoing elective surgery, randomly assigned to receive symbiotics or placebo for five days prior to the surgical procedure and for 14 days after surgery. We studied 91 patients, 49 in the symbiotics group (Lactobacillus acidophilus 108 to 109 CFU, Lactobacillus rhamnosus 108 to 109 CFU, Lactobacillus casei 108 to 109 CFU, Bifi dobacterium 108 to 109 CFU and fructo-oligosaccharide (FOS) 6g) and 42 in the placebo group. RESULTS surgical site infection occurred in one (2%) patient in the symbiotics group and in nine (21.4%) patients in the control group (p=0.002). There were three cases of intraabdominal abscess and four cases of pneumonia in the control group, whereas we observed no infections in patients receiving symbiotics (p=0.001). CONCLUSIONS the perioperative administration of symbiotics significantly reduced postoperative infection rates in patients with colorectal cancer. Additional studies are needed to confirm the role of symbiotics in the surgical treatment of colorectal cancer.