OBJECTIVE This study investigates the effects of hyperbaric oxygen (HBO2) therapy on the healing capacity of colonic anastomoses under the influence of preoperative chemotherapy. METHODS Forty male Wistar-Albino rats were divided into four groups of 10. Colonic resection and anastomosis were performed in each group. Group I (control) received no further treatment. In group II, 5-fluorouracil was administered intraperitoneally for five consecutive days preoperatively. Group III received HBO2 therapy for seven days after the anastomosis. Group IV received HBO2 therapy following the administration of preoperative 5-fluorouracil. On the seventh postoperative day, all the rats were examined to determine the bursting pressures of the anastomosis and to take tissue sample from the anastomotic line for hydroxyproline measurement. RESULTS Bursting pressures of the anastomosis in group IV were increased significantly compared to group II. Hydroxyproline levels were significantly increased with the use of HBO2 in rats, independent of chemotherapy administration. CONCLUSIONS HBO2 therapy strengthens anastomoses created under the influence of neoadjuvant chemotherapy. This technique might have a future role in the care of colon cancer patients undergoing new multimodality cancer treatments.