Hypoparathyroidism is among the few hormonal insufficiency states not treated with replacement of the missing hormone. This is the first randomized controlled study in children comparing treatment with synthetic human PTH 1-34 and calcitriol. The primary objective was to assess the efficacy and safety of long-term PTH 1-34 vs. calcitriol treatment in the maintenance of normal serum calcium values and renal calcium excretion in children with hypoparathyroidism. The study was conducted at a clinical research center. Subjects included 12 children aged 5-14 yr with chronic hypoparathyroidism and without severe renal or hepatic insufficiency. The study was a 3-yr randomized parallel trial comparing twice-daily calcitriol (plus calcium and cholecalciferol in four daily doses) vs. s.c. PTH 1-34 treatment, with weekly or biweekly monitoring of serum and urine calcium. Mean predose serum calcium levels were maintained at, or just below, the normal range, and urine calcium levels remained in the normal range throughout the 3-yr study, with no significant differences between treatment groups. Creatinine clearance, corrected for body surface area, did not differ between groups and remained normal throughout the study. Markers of bone turnover were mildly elevated during PTH 1-34 therapy and remained within the normal range during calcitriol therapy. Mean bone mineral density Z-scores at the anterior-posterior lumbar spine, femoral neck, distal radius, and whole body remained within the normal range and did not differ between groups throughout the study. Similarly, height and weight percentiles did not differ between treatment groups and remained normal throughout the 3-yr follow-up. We conclude that PTH 1-34 therapy is safe and effective in maintaining stable calcium homeostasis in children with hypoparathyroidism. Additionally, PTH 1-34 treatment allowed normal skeletal development because there were no differences in bone mineral accrual, linear growth, or weight gain between the two treatment arms over the 3-yr study period.