The hallmark of acute hypocalcemia (ionized calcium <0.75 mmol/l) is tetany, which is characterized by neuromuscular irritability. The symptoms may be mild with circumoral numbness, paresthesias of hands and feet, and muscular cramps or severe with laryngospasm, focal or generalized tonic muscle cramps, or seizures. Myocardial dysfunction and prolongation of QT interval also may occur. Most often, acute hypocalcemia occurs after thyroid or parathyroid surgery. Rarer cases are intravascular binding of ionized calcium by phosphate, citrate, or drugs such as foscarnet or bisphosphonates. The most appropriate treatment is intravenous calcium, in the form of 100-200 mg of elemental calcium. Thereafter, the therapy depends on the underlying disease. In most cases vitamin D has to be added to calcium substitution. In cases of hypomagnesemia, magnesium and not calcium has to be substituted. It has not yet been proven in clinical trials whether substitution of magnesium and/or calcium influences the clinical outcome in patients with severe sepsis or pancreatitis who show both hypomagnesemia and hypocalcemia.