Low red-cell concentrations of 2,3-diphosphoglycerate (2,3-DPG) and adenosine triphosphate (ATP) with resulting increased oxygen affinity of hemoglobin is one of the mechanisms by which hypophosphatemia exerts clinically detrimental effects. The possibility that obligatory post-traumatic and/or glucose-induced hypophosphatemia leads to changes in red-cell 2,3-DPG and ATP concentrations was investigated in patients undergoing elective colonic surgery. In addition to standardized intravenous fluids postoperatively, one (randomized) patient group received continuous glucose-saline infusion, which is specially associated with hypophosphatemia, while another group was given the same amount of glucose/24 hours, but in a 5-hour infusion. Serum phosphate fell significantly in both groups, mainly in the former, but unaccompanied by change in red-cell 2,3-DPG or ATP concentration. During the 5-hour glucose infusions significant hypophosphatemia appeared, but without reduction of 2,3-DPG or ATP. Thus neither the obligatory hypophosphatemia after elective surgery nor the further fall during postoperative glucose infusion altered the red-cell concentration of 2,3-DPG or ATP.