The effect of an increasing serum ionized calcium concentration on blood pressure during hemodialysis was studied in a prospective cross-over trial. Twelve patients were dialyzed three times with a dialysate Ca concentration of 5.5 mg/dL and three times with one of 7.5 mg/dL. Dialysis with the high Ca dialysate was associated with less fall in the mean arterial pressure (p less than 0.001). No statistically significant changes occurred in serum osmolality, body weight, or heart rate after dialysis at the two dialysate Ca levels. Increasing the Ca concentration in dialysate may protect against intradialytic hypotension in some patients.