The effects of intravenous administration of potassium phosphate in the treatment of diabetic ketoacidosis were studied in nine children, ages 9 9/12 to 17 10/12 yr. During phosphate infusion (20--40 meq/L of fluid), all children maintained normal serum concentrations of phosphorus. Transient hypocalcemia occurred in six and transient hypomagnesemia in five patients. One child developed carpopedal spasms refractory to intravenous infusion of calcium gluconate but responsive to intramuscular injection of magnesium sulfate. In three patients, serum levels of intact parathyroid hormone were low at the time of hypocalcemia, an observation that suggests transient hypoparathyroidism. This study indicates that the use of potassium phosphate as the sole source of potassium replacement might potentiate ketoacidosis-induced hypocalcemia through multiple mechanisms.