Among 15 patients with PTH-deficient idiopathic hypocalcemia, we found two novel missense mutations in the calcium-sensing receptor (CaSR). Patient 1, who developed severe hypocalcemia (5.0 mg/dL) and seizures after birth, had a heterozygous de novo missense mutation in the transmembrane domain (A844P). The patient is currently receiving a minimum dose of 1alpha-OHD(3) (0.5 microg/day) to maintain the serum calcium level at 6 mg/dL and thus prevent seizures. Patient 2 had asymptomatic hypocalcemia (7.5 mg/dL) and also had a heterozygous missense mutation in the extracellular domain (E228G). These findings suggest that gene analysis of CaSR should be performed in patients with idiopathic hypocalcemia, particularly when it occurs in the neonatal period.