Hypophosphataemia in connection with surgical trauma is most commonly ascribed to parenteral nutrition, glucose infusion, or haemodilution. We have earlier shown that patients with severe burns develop hypophosphataemia irrespective of such causes, and have introduced the theory that hypophosphataemia can result from the humoral response to trauma. In the present work, 10 otherwise healthy patients undergoing elective surgical treatment of old knee-ligament injuries were examined pre-, per-, and postoperatively with regard to phosphate, calcium, and magnesium homeostasis and hormones regulating this (parathyroid hormone, calcitonin, catecholamines). During operation we noted a significant and rapid fall in serum phosphate concentration, accompanied by increased secretion of calcitonin and catecholamines and a significant fall in serum magnesium. Unchanged fractional and total urinary phosphate excretion indicated that the fall in serum phosphate concentration was mainly due to prerenal causes.