A considerable body of data has associated lower blood pressures with higher levels of dietary calcium. Epidemiologic studies, laboratory evaluations, and clinical trials have also indicated that the incidence of hypertensive disorders of pregnancy is affected similarly by calcium intake. A meta-analysis of five controlled clinical trials of calcium supplementation in pregnancy suggested a significant reduction in proteinuric pre-eclampsia of 46 percent. Several of the trials, however, suffered from 'pitfalls' in the diagnosis of pre-eclampsia, including lack of blinding, uncertain definition of endpoints, and unknown techniques of measurement. Most trials have not assessed the role of dietary nutrients or the possibility that a subgroup with low baseline urinary calcium may benefit most from calcium supplementation. In no trial has the potential for increased risk of kidney stones in the treatment group been examined systematically. Moreover, the daily schedule for administration of calcium, has generally not been reported. There was a great need, therefore, to evaluate the efficacy of calcium supplementation for the prevention of pre-eclampsia in a large multicenter controlled clinical trial. The trial considered the role of dietary nutrients, establish whether treatment is beneficial only for those with low baseline urinary calcium, conduct systematic surveillance for urolithiasis, and employ standardized terminology, techniques of measurement, and diagnostic criteria. The NHLBI provided funding to NICHD for three years by means of an Intraagency Agreement (Y01HC20154).