In 33 severely burned patients, who were followed for two weeks after injury, a transient hypophosphatemia was registered with the decrease in serum phosphate occurring on the second to tenth day after injury. When a previous retrospective study was combined with the present one, there were 50 patients in all. Seven patients died and, of these, five showed very low values of serum phosphate (S-P) at the time of their death. Simultaneous reduction of urinary phosphate excretion indicates that the depletion of phosphate is mainly prerenal. However, studies of fractional excretion of phosphate demonstrates that renal losses of phosphate might contribute to the hypophosphatemia. A remarkable finding was a general, marked rise in serum calcitonin (S-h-CT). The relationship between the rise in S-h-CT, the increased catecholamine secretion and the hypophosphatemia remains to be further clarified.