To determine the effect of estrogen on vitamin D metabolism in pubertal girls, we studied 16 tall girls treated with a daily dose of 4-8 mg estradiol valerate to curtail excessive adult height. In all but one girl the plasma concentration of 1,25-dihydroxyvitamin D (1,25-(OH)2D) increased to values significantly higher than the corresponding pretreatment value (P less than 0.0005). The ratio of 24,25-dihydroxyvitamin D (24,25-(OH)2D) to 25-hydroxyvitamin D decreased in all girls (P less than 0.0005). The vitamin D binding protein (DBP) also increased significantly after estrogen (P less than 0.025), and there was a significant positive correlation between the plasma concentration of 1,25-(OH)2D and DBP (r = 0.66; P less than 0.0005). The free fraction of 1,25-(OH)2D remained unchanged after estrogen. It appears that estrogen treatment increases the plasma concentration of 1,25-(OH)2D. The effect might be explained by the concomitant increase in DBP and/or by estrogen stimulation of renal 1 alpha-hydroxylase.