The first thing to do is to confirm that the number of circulating calcium ions has really decreased. This is achieved either by correction of the total blood calcium level, taking into account possible variations in albuminaemia and pH, or by direct measurement of plasma calcium ions, using a special electrode. The aetiological diagnosis may be easy in some clinical situations, but it often demands a systematic approach, which implies a specialized and brief exploration, feasible in out-patients, with simultaneous measurement, under basal conditions, not only of plasma calcium ions, but also of plasma magnesium, intact 1-84 parathyroid hormone (PTH), nephrogenic cyclic AMP and 25 (OH) D. Rationally, hypocalcaemias may be divided into two groups : (1) extraparathyroid, where hyperparathyroidism is constant and hypocalcaemia is due either to calcium intake reduction, vitamin deficiency of high bone accretion, or to a primary renal calcium leakage; (2) parathyroid, by impaired secretion of PTH or alteration of PTH receptors, which means hypoparathyroidism or pseudohypoparathyroidism. The diagnostic and therapeutic possibilities in both groups are discussed.