The concentrations of serum inorganic phosphate complexes (HPO4(2-), H2PO4-, NaHPO4-, KHPO4-, CaHPO4 and MgHPO4) were studied during diabetic ketoacidosis. On admission to hospital the H2PO4- concentration was significantly increased, in comparison with well-controlled diabetics. After 12-24 h the concentration of all phosphate complexes decreased. At 72 h only the MgHPO4 concentration was decreased. These changes are related to acidosis, insulin therapy and tubular dysfunction.