The effects of total-body hyperthermia on phosphorus homeostasis are controversial. To evaluate the problem, 10 clearance studies were performed in seven patients undergoing total-body hyperthermia as an adjunct to the treatment of solid malignant tumors. Total-body hyperthermia was associated with significant reduction in plasma phosphorus concentration from a baseline value of 3.51 +/- 0.18 to 0.6 +/- 0.1 mg/dl (p less than 0.001), returning to baseline following cessation of total-body hyperthermia. The clearance of phosphorus increased from 15.2 +/- 2.5 to 26.1 +/- 3.1 ml per minute (p less than 0.01), and the fractional excretion of phosphorus increased from 11.37 +/- 2.2 to 47.68 +/- 9.7 percent (p less than 0.01). The reduction in plasma phosphorus during total-body hyperthermia was also associated with a significant reduction in the renal threshold phosphorus concentration from 3.17 +/- 0.16 to 0.38 +/- 0.08 (p less than 0.001). The changes in phosphorus homeostasis during total-body hyperthermia were independent of changes in circulating parathyroid hormone level, urinary cyclic AMP excretion, and arterial carbon dioxide tension.