The rate of mass-transfer (MT) of magnesium during hemodialysis was studied in thirty-five patients with hypermagnesemia (Mg = 3.75 +/- 0.72 mg/dl) undergoing chronic hemodialysis. The aim of the study was to verify which is the best dialytical approach to remove the excess of magnesium. The concentration of Mg in the dialysate was of 1.82 mg/dl for all patients. MT was -0.51 +/- 0.36 g and no statistical difference was found between patients treated with cuprophan hollow fibers dialyzers, PAN and cuprophan plates. Mg MT is not correlated with dialysis duration (r = -0.23; p:ns), urea clearance (r = -0.08; p:ns), KT/V index (r = -0.03; p:ns), blood flow (r = -0.15; p:ns). In conclusion from our data, in agreement with other Authors, reduction of serum Mg levels is more convenient by obtained by a decrease in Mg concentration in the dialysate under 1.82 mg/dl, in order to increase the blood-dialysate concentration gradient.