Evidence exists of the important role of sodium balance and extracellular fluid volume in the genesis of hypertension in chronic renal failure (CRF). Several studies have shown that patients with advanced CRF have an increased, interstitial and intravascular, extracellular water volume (ECW). The relationship between the increase in ECW and high blood pressure has also been reported. Using electric bioimpedance, body water distribution was studied in 32 patients from our dialysis unit. Twelve of these patients were hypertensive and 20 had normal blood pressure. Hypertensive patients had a significantly greater total body water volume and ECW than the normotensive patients. Given the importance of ECW in controlling blood pressure, one of the main aims of haemodialysis is a suitable extraction of sodium and water during the treatment. Despite technological advances in dialysis therapy, cardiovascular instability during treatment is still a clinical problem. In recent years, new strategies to control ECW, with good haemodynamic tolerance, have been developed. These strategies include haemofiltration, haemodiafiltration and sodium and ultrafiltration profiles.