Urine calcium excretion was evaluated in 19 patients before and after calcitriol (1,25-dihydroxyvitamin D3) treatment that was followed up for a five- to seven-year period. The effects of increases of calcitriol dosage and modifications of calciuria with hydrochlorothiazide were systematically examined. The urine calcium excretion before calcitriol therapy was 2.3 +/- 0.8 mg/kg/day (mean- +/- SEM) and the urine calcium-creatinine concentration ratio was 0.12 +/- 0.04. With the dose of calcitriol at 23 ng/kg/day, these values increased to 3.2 +/- 0.8 mg/kg/day and 0.19 +/- 0.04, respectively. Following double dose of calcitriol (44 ng/kg/day), increments in calciuria and urinary calcium/creatinine concentration of 1.4 +/- 0.6 mg/kg/day and 0.10 +/- 0.03, respectively, were observed. With concomitant administration of hydrochlorothiazide (1 to 2 mg/kg/day) therapy at maintenance dose and calcitriol (31 ng/kg/day), the urine calcium excretion effectively decreased by 1.3 +/- 0.6 mg/kg/day and the urine calcium-creatinine concentration ratio by 0.05 +/- 0.02. The results suggest that children with calcium-phosphate disorders who require long-term treatment with calcitriol must be carefully monitored in terms of urine calcium excretion, especially when the dosages are increased to achieve maximal therapeutic efficacy. Calciuria induced by calcitriol administration is effectively reversed by addition of hydrochlorothiazide to the treatment regimen.