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OBJECTIVE To evaluate the role of intravenous fluid volume and electrolyte supplementation on the development of postoperative reflux (POR) in horses undergoing celiotomy for colic. DESIGN Case-control study spanning 2004-2012 for horses undergoing celiotomy for colic. SETTING University teaching hospital. ANIMALS Sixty-seven client-owned horses >1 year of age with POR were each matched to 2 controls with similar surgical lesions that did not demonstrate POR. MEASUREMENTS AND MAIN RESULTS Survival was significantly lower in cases (65.7%) than controls (96.1%). Factors found to be associated with POR included decreased net fluid volume administered on day 1 postoperatively, increased age, and performing a resection and anastomosis. Mean time until onset of POR was 20.4 hours postoperatively. PCV was significantly higher immediately following surgery and at 24 hours postoperatively in horses that developed POR compared with matched controls. There was no association between electrolyte values at presentation or administration of potassium, calcium, or magnesium in the postoperative period and the subsequent development of reflux. CONCLUSIONS In the perioperative period, IV fluid volume overload and electrolyte abnormalities were not contributing factors in the development of POR in this population of surgical colic patients. Close postoperative monitoring with consideration and correction of pre- and postoperative fluid deficits is recommended.