Plasma biochemistry, bone radiology and morphometry were studied in a group of 20 patients receiving maintenance hemodialysis. The aim was to determine if increasing the dialysate calcium concentration would decrease plasma parathyroid hormone and improve the radiologic appearances of bone in patients without producing serious side effects. Dialysate calcium concentration was increased stepwise from 4.5 to 6.0 and then to 7.0 mg/100 ml. Mean predialysis plasma calcium concentration increased from 9.4 to 9.7 and then to 10.0 mg/100 ml and mean predialysis phosphate concentration increased from 5.3 to 5.6 mg/100 ml. Parathyroid hormone concentration was elevated in all patients but the mean concentration did not change significantly although in seven patients a decrease occurred. Six patients had radiologic signs of renal bone disease, two patients showed improvement and three patients developed bone disease during the study. The patients with radiologic bone disease had the highest parathyroid hormone concentrations and the majority were female. Morphometric bone measurements showed that bone loss during the study occurred mainly in the male patients. Soft tissue calcification continued to appear during the study. Although the response to some patients to an increased dialysate calcium concentration was favorable, it was impossible to predict which types of patient would benefit from the use of a dialysate calcium concentration of 7.0 mg/100 ml.