Tetany was observed in a 18 months old girl, suffering from severe diarrhea. Even continuous iv calcium administration did not prevent tetanic spasms. Further evaluation revealed hypomagnesemia, hypocalcemia, and functional hypoparathyroidism. Administration of MgSO4 normalized serum levels of Mg and raised iPTH levels; tetany subsided. Serum calcium and P levels were normalized later. The pathogenesis of hypocalcemia secondary to magnesium deficiency might be a functional hypoparathyroidism as well as an endorgan-resistance to PTH.