An antimicrobial agent trimethoprim-sulfamethoxazole (Tmp-Smx)does not usually cause electrolyte disturbances at regular doses, and few cases ofTmp-Smx-inducedelectrolyte imbalance have been reported in the English-language literature to date. Recently, however, we treated two patients with Pneumocystis carinii pneumoniawhodeveloped severe hyponatremia and hyperkalemia on administration of high-dose Tmp-Smx.These electrolyte disturbances were attributable to the direct effect of Tmp-Smxon the renal distal tubules, were reversible, and corrected by infusion of a sodium-enriched and potassium-free liquid. Therefore, it is suggested that even after electrolyte disturbances have occurred, high-dose Tmp-Smxtherapy may be continued for severe infectious diseases under appropriate electrolyte correction. (Internal Medicine 34: 96-99, 1995)