In the international and the Hungarian guidelines, the diuretics in the first line of the treatment of hypertension. Their sometimes false judgment is based on the side effects, because of the over dosage of the applied medication. According to finished studies, efficiency of thiazides is usually the same as that of their competitors in influencing of the cardiovascular morbidity and mortality. Thiazides have to be given in the first line an antihypertensive treatment to the patients, especially if they are old, or have a great risk for a cardiovascular complication (stroke, coronary heart disease, heart failure, left ventricular hypertrophy). In the case of natrium-retention (diabetes, obesity, nephropathy), the treatment without diuretics is not effective. Thiazides make stronger the effects especially of ACE-inhibitors, angiotensin receptor blockers, and beta receptor blockers. The newer diuretics--with fewer side effects--have very likely extrarenal way of effects, so their long time application seems very favourable.