During treatment with low to moderate doses of thiazides or loop diuretics, hypokalemia is dose-dependently demonstrated in 2-11% of patients. Additional hypomagnesemia is present in about 40% of hypokalemic patients. High doses of diuretics were routinely used in the past for treatment of hypertension or heartfailure, causing ventricular arrhythmias and sudden cardiac death. Low-dose thiazides +/- potassium-sparing diuretic are not associated with these severe adverse effects, but improve in contrast (to high-dose diuretics) survival and cardiovascular morbidity in hypertensive patients. Higher doses of diuretics often necessary in the treatment of patients with severe heart failure are today no longer regularly associated with hypokalemia/hypomagnesemia because of the concomitant treatment with an ACE inhibitor and low-doses of spironolactone.