A 77-year-old male was admitted to hospital after suffering a pelvic bone fracture in a road traffic accident and was incidentally found to have IgG-kappa-type multiple myeloma with hypercalcemia. The patient was also noted to be hypokalemic and had low HCO3 (-), with possible damage to the distal tubules in the kidneys. When the treatment was begun with bortezomib/dexamethasone/elcatonin and sodium bicarbonate (NaHCO3) in normal saline (equivalent to a daily sodium dose of 200 millimoles per liter [mmol/L]), the patient was in a state of poor oral fluid intake. The patient developed hypernatremia and hyperchloremia, with a peak serum sodium and chloride levels of 183 mmol/L and 153 mmol/L, respectively, at the sixth day after the start of treatment. Following the switch of the intravenous infusions from normal saline to soldem 1 and soldem 3 solutions, these high-electrolyte levels gradually returned to normal over the next 7 days. Although the patient showed disturbed consciousness (Japan Coma Scale = JCS-I-3) during the period of electrolyte abnormality, he eventually fully recovered without sequelae. In this patient, we successfully managed the severe hypernatremia/hyperchloremia, caused by the combined effects of intravenous saline burden in a state of poor oral fluid intake, during the treatment for IgG-kappa type multiple myeloma.