Relative hypocalciuria has been reported in women with preeclampsia. However, there has been no systematic explanation for this finding. We measured serum and urinary calcium and serum calciotropic hormones in third trimester women with preeclampsia (n = 12, gestational hypertension and proteinuria) and with normotensive pregnancies (n = 24) to try to explain these changes. We confirmed that the women with preeclampsia have a relative hypocalciuria (2.9 +/- 0.7 vs. 6.5 +/- 0.2 mmol/day, P less than 0.01). Preeclamptic women also had lower serum ionized calcium than normotensive third trimester pregnant women (1.20 +/- 0.01 vs. 1.26 +/- 0.01 mmol/L, P less than 0.02). Intact PTH levels were significantly higher in preeclamptic women (29.9 +/- 4.3 vs. 15.4 +/- 1.3 ng/L, P less than 0.01) and a significant inverse relationship was observed between PTH and both urine calcium (r = -0.60, P less than 0.0001) and serum ionized calcium (r = -0.36, P less than 0.05). We measured vitamin D metabolites in a subgroup of both normotensive and preeclamptics. Preeclamptic and normotensive pregnant women had equivalent levels of 25-hydroxyvitamin D [25(OH)D]; however, preeclamptics had significantly lower 1,25-dihydroxyvitamin D [1,25-(OH)2D] levels (172.1 +/- 18.5 vs. 219.6 +/- 12.7 pmol/L, P less than 0.05). Lower 1,25-(OH)2D may contribute to suboptimal intestinal absorption of calcium during a time of increased calcium demand resulting in lower ionized calcium, increased PTH, and hypocalciuria in preeclampsia. Abnormalities in calcium homeostasis may contribute to the increased vascular sensitivity documented in preeclampsia.