Recent studies have suggested that preeclampsia is associated with hypocalciuria. We compared 24-hour urine data from 143 obstetric patients: 33 with preeclampsia, 58 normotensives, and 52 patients with gestational hypertension. The mean maternal age, race, and parity of these three groups did not differ significantly. The patients with preeclampsia had significantly less excretion of total calcium (129.7 +/- 18.7 mg/24 hours) (mean +/- SEM) than normotensives (283.9 +/- 12.3) or those with gestational hypertension (233.2 +/- 22.3) (P = .0001). The mean arterial pressure and excretion of total protein were significantly increased in the preeclampsia group (P = .0001). Using the receiver operator curve, a urine calcium threshold of 12 mg/dL was chosen as predictive for the development of preeclampsia, with a sensitivity of 85%, specificity of 91%, and positive and negative predictive values of 85 and 91%, respectively. Urinary calcium levels below 12 mg/dL may help distinguish preeclampsia from other hypertensive disorders of pregnancy.