In 140 patients with essential hypertension effectiveness of hypotensive treatment was assessed clinically by the results of single measurements of arterial pressure (AP) vs results of 24-h AP monitoring. According to 24-h monitoring, effectiveness of hypotensive treatment was clinically overestimated because night AP lowering was not sufficient and plasm sodium levels were relatively high. In 20.0% of the patients treatment results were, on the contrary, underestimated as shown by marked pressor reactions and relative hypopotassemia. Thus, single measurements of AP provide wrong results of antihypertensive treatment in one-third of the patients. Plasma levels of sodium and potassium can predict the error.