Eleven patients with diabetic ketoacidosis were given intravenous phosphate in doses (mean 118 mmol; range 83--320 mmol) adequate to maintain normal plasma phosphate, in addition to a standard treatment regime. Prevention of hypophosphataemia stimulated recovery of the initially low red-cell 2,3-diphosphoglycerate concentrations (10.6 +/- 5.8 (SD) mumol/g Hb) after twenty-four hours. In ten control patients (initial concentration 8.1 +/- 4.4 mumol/g Hb) treated without phosphate replacement, significantly lower red-cell 2,3-diphosphoglycerate concentrations were found between 2 and 6 days after admission (forty-eight hour value for control patients 14.6 +/- 1.6 and for phosphate-treated patients 18.9 +/- 4.1 mumol/g Hb; p less than 0.01). However, no effect on in vivo p 50 or on the availability of oxygen from the blood resulted from the higher 2,3-diphosphoglycerate levels. Maintenance of normal plasma phosphate levels by intravenous phosphate is, therefore, not indicated to improve tissue oxygenation in diabetic ketoacidosis.