Pre- and post-dialysis plasma calcium concentrations have been surveyed and correlated with known changes in dialysate calcium concentrations during 7 years of maintenance haemodialysis. When dialysate calcium levels were 7 or 8 mg/ 100 ml, the patients were hypercalcaemic. Plasma calcium fluctuated within the normal range when the dialysate calcium was maintained around 6 mg/ 100 ml. Hypercalcaemia during maintenance dialysis may indicate only that the selected dialysate calcium is inappropriately high. The highest dialysate calcium which enables the pre- and post-dialysis plasma calcium levels to remain within the normal range is recommended. Even in traditionally soft water areas the effects of possible changes in the calcium content of the mains water must be considered.