Urinary calcium excretion decreases in preeclampsia. To determine the predictability of preeclampsia with hypocalciuria, we investigated the calcium to creatinine ratio and the calcium level of a spot urine sample in 56 primigravid patients aged less than 25 years. Of these 56 cases, 44 remained normotensive and preeclampsia developed in eight cases. There were four cases of gestational hypertension and they are not included in the statistical analysis. The mean age, the mean gestational age at entrance into the study, the mean urine calcium concentration, the mean birthweight, and the mean gestational age at delivery were similar between the normotensive and preeclamptic groups. The mean calcium to creatinine ratio is found to be significantly lower in the preeclamptic group (0.0475 +/- 0.0260) compared with the normotensive group (0.1466 +/- 0.1353; p < 0.0001). A cutoff level of 0.066 for the calcium to creatinine ratio with the use of a receiver operator curve yielded a sensitivity of 75%, a specificity of 86%, and a positive and negative predictive value of 55% and 95%, respectively. The results of this study suggest that a single urine calcium to creatinine ratio might be an effective marker for predicting preeclampsia in a high-risk population.