Early neonatal hypocalcemia occurs in premature infants, infants with birth asphyxia, and infants of diabetic mothers. Etiological considerations include decreased calcium (Ca) supply, increased endogenous phosphate load, hypomagnesemia, alkali therapy, functional hypoparathyroidism, defects in vitamin D metabolism, and possibly calcitonin excess. Late neonatal hypocalcemia occurs, with malabsorption of magnesium (Mg), increased exogenous phosphate loading, after alkali therapy of diarrheal acidosis, hypoparathyroidism, and vitamin D related disorders. The therapy of hypocalcemia includes oral or iv Ca salts and in the near future, possibly the newly discovered vitamin D metabolites.