Severe trauma is associated with increased expenditure of energy, which leads to heightened cellular requirements for regeneration of high energy phosphates (HEP). The effect of multiple traumata and of burns (18 and 12 patients, respectively) on serum phosphate (S-P) and muscle phosphate metabolism was studied. Blood and muscle samples were obtained 2, 4, 8 and 30 days after the injury. The results of analyses were compared with findings in 14 non-traumatized controls. S-P showed a decrease 2 and 4 days after the injury, despite a phosphate supply of 0.4 mmol/kg BW/day. Reduction of HEP and inorganic phosphate of muscle tissue (Pi) was observed on post-trauma days 2, 4, 8 and 30. No correlation was found between Pi and S-P. The fall in S-P, HEP and Pi was not affected by increasing the phosphate supply from 0.4 to 1.0 mmol/kg BW/day. In patients older than 60 years, the reduction of S-P, and adenosine triphosphate was greater than in the younger patients. The results suggest that the S-P decrease was due to loss via the urine and a shift from extracellular to intracellular compartment. The fall in Pi and HEP probably was caused by impaired ability of the cells to utilize the phosphate available in serum, and direct cellular loss of phosphate resulting from hypermetabolism and catabolism.