The human body is equipped with an efficient protection system against hypocalcemia. This system is composed of parathyroid glands, bone, kidney, and intestine. By appropriate actions of parathyroid hormone (PTH) and active vitamin D (1,25-dihydroxyvitamin D), a small fall of extracellular calcium ion concentration is instantly corrected. Thus, a defect of any step in this system results in the development of hypocalcemia. Overloaded calcium either from bone or intestine is efficiently excreted into the urine. Thus, hypercalcemia develops almost exclusively when a greater amount of calcium than the kidney can excrete is loaded. In chronic dialysis patients, either hypocalcemia or hypercalcemia may develop because of defects in these two defense systems against calcium imbalance.