© 2017 by the Endocrine Society. Context: Hypercalciuria is an adverse event of postsurgical hypoparathyroidism treatment that can lead to renal complications. The collection of 24-hour urine to detect hypercalciuria is often considered unreliable. Objective: The purpose of this study was to find useful predictive biomarkers of hypercalciuria in patients with permanent postsurgical hypoparathyroidism receiving treatment with oral calcium and calcitriol supplements. Design and Setting: The investigation was designed as a prospective cross-sectional study. An outpatient hospital clinic served as the study setting. Patients: Fifty-four consecutive observations were made of 34 stable outpatients with postsurgical hypoparathyroidism taking oral calcium and calcitriol supplements, and 17 adult controls without hypoparathyroidism. Intervention: There were no interventions. Main Outcome Measure: Hypercalciuria was defined as 24-hour urine calcium >300 mg. Results: Patients without hypercalciuria (n = 21) vs those with hypercalciuria (n = 33) had lower levels of serum 1,25-dihydroxyvitamin D (33.5 ± 11.9 pg/mL vs 45.8 ± 9.5 pg/mL; P < 0.001), similar albumincorrected serum calcium (8.3 ± 0.5 vs 8.6 ± 0.5 mg/dL; P = nonsignificant), and serum parathyroid hormone (12.5 ± 5.7 vs 10.7 ± 6.8 pg/mL; P = nonsignificant).Multiple linear regression analysis showed an independent relationship between 1,25-dihydroxyvitamin D and urinary calcium excretion (B = 6.26 1.423; P<0.001). A cutoff value of 33.5 pg/mL for serum 1,25-dihydroxyvitamin D to predict the absence of hypercalciuria had 100% sensitivity and 63.6% specificity, and the area under the receiver operating characteristic curve was 0.797. No patients with serum 1,25-dihydroxyvitamin D levels of <33.5 pg/mL presented with hypercalciuria, regardless of the level of albumin-corrected serum calcium. Conclusions: Routine measurement of serum 1,25-dihydroxyvitamin D may be useful as a biomarker to predict the absence of hypercalciuria in patients with permanent postsurgical hypoparathyroidism who are receiving treatment with oral calcium and calcitriol supplements.