A controlled double-blind prospective study was undertaken of the effect of dialysate calcium levels on BP during hemodialysis. Twenty patients and 240 dialyses were studied using a protocol in which patients underwent alternate hemodialyses with dialysate calcium of 2.5 and 3.5 mEq/L. Dialysate composition was otherwise the same. Mean BPs during dialysis were significantly lower at 1.5, 2.5, and 3.5 hours of dialysis when the lower dialysate calcium was used (P = .007 to .02). However, the difference in BP between the high and low dialysate calcium treatments was clinically minor, with a maximum mean difference (at 1.5 hours) of 4.6 mm Hg. Subgroups of patients with frequent hypotension and low or normal serum calcium did not appear more sensitive to the hypotensive effect of low calcium dialysate. Dialysate calcium levels of 2.5 and 3.5 mEq/L thus differ in their effect on intradialytic BP in a statistically significant, but clinically minor, way. Low calcium dialysate thus may prove useful in the management of patients in whom large amounts of enteric calcium absorption are indicated or unavoidable.