['In some haemodialysis patients, blood pressure increases during the dialysis session despite ultrafiltration (UF).', 'We investigated six such patients who were not responsive to hypotensive drugs. Their echocardiograms were obtained prior to and during the dialysis session.', 'After the mean 2520 +/- 1698 (4.5 +/- 2.3% of BW) ml of fluid was removed, the cardiac systolic function parameters significantly improved and maximum mean arterial pressure rose (from 107 +/- 5 to 118 +/- 6 mmHg, p < 0.027). This increase in blood pressure was accompanied by an increase in cardiac index (from 3.8 +/- 0.6 to 4.8 +/- 1.1 L/min/m2, p < 0.027). With continuing UF, after a mean fluid removal of 4133 +/- 1622 (7.5 +/- 2.1% of BW) ml, normal blood pressure was achieved in all patients. Previously increased ejection fraction and fractional shortening decreased. End-diastolic volume significantly decreased from 98 +/- 34 to 78 +/- 35 ml/m2 indicating normovolemia. This decrease in blood pressure was accompanied by a return of cardiac index to normal values (from 4.8 +/- 1.1 to 3.1 +/- 0.8 L/min/m2, p < 0.027). There was a positive correlation between mean arterial pressure and cardiac index (r = 0.56, p = 0.017).'] Is paradoxical rise in blood pressure during ultrafiltration caused by increased cardiac output?