Gain-of-function mutations in the calcium ion-sensing receptor (CaR) cause hypocalcemia with low PTH levels. It is stated that patients with activating CaR mutations generally show milder degree of hypocalcemia before treatment and more profound hypercalciuria during treatment than those with idiopathic hypoparathyroidism (IHP). To test this validity we analyzed the data of serum and urinary calcium collected from 85 patients with IHP and 15 with activating CaR mutations. The mean (±sem) serum calcium concentration before treatment was significantly higher (P < 0.001) in patients with activating CaR mutations (1.76 ± 0.05 mmol/L; n = 15) than in those with IHP (1.41 ± 0.03; n = 58), but there was a substantial overlap in the range of hypocalcemia between the two groups (1.25–2.05 vs. 0.90–1.95). The mean urinary calcium/creatinine ratio (Ca/Cr) in patients with activating CaR mutations before treatment (0.362 ± 0.045 mmol/mmol; n = 9) was almost equal to that in normocalcemic controls (0.331 ± 0.022; n = 56) and markedly higher (P < 0.001) than in patients with IHP (0.093 ± 0.008; n = 57). The overlap of pretreatment urinary Ca/Cr between the 2 disorders was relatively small; subnormal urinary Ca/Cr was observed in only 1 of 9 patients with CaR mutations and in the majority (49 of 57) of patients with IHP. In contrast to pretreatment findings, the degree of hypercalciuria during treatment was not different between the 2 disorders. The data points of urinary Ca/Cr plotted as a function of the serum calcium concentration were not separable between patients with CaR mutations (n = 8) and those with IHP (n = 40). Comparison of urinary Ca/Cr between 2 patients with a CaR mutation and 7 with IHP over a wide range of serum calcium concentrations measured during 4–8 yr of treatment also indicated that the 2 disorders were inseparable. These results suggested that inappropriately normal urinary Ca/Cr in patients with untreated hypocalcemia, mostly of mild degree, might be a better biochemical clue than the development of severe hypercalciuria during treatment to suspect gain-of-function mutations in the CaR.