A significant proportion of patients formerly diagnosed with idiopathic hypoparathyroidism actually have activating mutation of the calcium-sensing receptor (CaSR) gene. Awareness of the possibility of activating mutation of CaSR gene in patients with sporadic idiopathic hypoparathyroidism is important because of its relevance to clinical management. This report is of a novel activating mutation of the CaSR gene identified in a 10-year-old Chinese girl who was initially diagnosed as having idiopathic hypoparathyroidism at 6 years of age after presenting with seizures. Her serum calcium level was difficult to maintain near the lower limit of normal despite treatment with high-dose calcitriol. Treatment with calcitriol produced significantly elevated urinary calcium-to-creatinine ratio. Direct sequencing of the CaSR gene showed a novel heterozygous mutation (p.Q735P (c.2204A>C)). Molecular genetic analysis of her parents demonstrated that both parents did not harbour the child's mutation, indicating that her mutation had arisen de novo.