Bacterial translocation, the passage of viable indigenous bacteria from the gastrointestinal tract to the mesenteric lymph nodes and other internal organs, has been poorly studied in man to date. Pericolonic lymph nodes, liver, portal blood, and peritoneum specimens were harvested before antibiotics were administered during 20 operations for colorectal cancer and compared with those obtained in 20 operations for non colorectal conditions. Bacterial translocation, defined as the presence of intestinal bacteria in at least one of the specimens, was found in 13 patients (65 percent) in the colorectal cancer group as compared to 6 (30 percent) in the control group (p less than 0.05). The increased incidence of bacterial translocation in colorectal cancers was mainly due to the presence of bacteria in the pericolonic lymph nodes adjacent to the cancer. These findings suggest that intestinal bacteria translocate from the bowel lumen in a high proportion of patients with colorectal cancer and further stress the need for prophylactic antibiotics in colorectal cancer surgery.