Most hemodialysis is now carried out with a dialysate sodium concentration of 140-145 mEq/L. Higher dialysate sodium has been used, but controversy exists concerning the increased incidence of high blood pressure (HBP), thirst, and weight gain. A double blind prospective study was carried out in five stable men on chronic hemodialysis. Dialysis was performed in random sequence with a dialysate sodium of 145, 150, or 155 mEq/L for 2 months at a time. Vital signs were monitored before, during, and after dialysis, and the presence of symptoms during and between dialyses was documented. There was a significant increase in interdialytic weight gain with increasing dialysate sodium: 145 mEq/L (2.2 kg), 150 mEq/L (2.6 kg), 155 mEq/L (2.9 kg). There was a small, nonsignificant increment in dry weight of 0.5 kg between a dialysate of 145 mEq/L to 155 mEq/L but no increase in the mean arterial blood pressure. There was no difference in the incidence of interdialytic or intradialytic symptoms, including cramps, nausea, or fatigue, nor any change in serum sodium or other routine laboratory data before dialysis. It is concluded that a high dialysate sodium is not associated with an increased incidence of hypertension, symptoms, or a change in serum sodium but is associated with an increase in interdialytic weight gain.