OBJECTIVE: To determine the serum and lymphocyte magnesium concentrations during normal pregnancy and to compare the magnesium status in the third trimester of pregnancy between women with normal pregnancy and those with gestational hypertension (GH) or pre-eclampsia (PE). DESIGN: A prospective cross-sectional study followed by a prospective comparative study. SETTING: Department of Obstetrics and Gynecology, Department of Pediatrics and Genetics, Hopital Cantonal Universitaire Geneve, Switzerland. SUBJECTS: Seventy-one healthy pregnant women, with normal pregnancies between 6 and 38 weeks gestation. The second part included 43 women in the third trimester of pregnancy, 11 had GH, 11 had PE and 21 formed the comparison group of healthy normotensive women. MAIN OUTCOME MEASURES: Total serum and intralymphocytic Mg concentrations and urinary Mg excretion. RESULTS: There was a progressive reduction in total serum magnesium concentrations during normal pregnancy, thought to be partly due to haemodilution, because the decline in concentration of serum proteins paralleled that of Mg (P less than 0.001). In the three groups studied in the third trimester the serum Mg concentration was very similar in the GH and the comparison groups, but it was significantly higher in the PE group (P less than 0.01). The intralymphocytic Mg concentrations and the urinary Mg excretion were similar in all three groups. In five patients treated with MgSO4 there was a large increase in the serum Mg concentration and in the urinary Mg excretion. The intralymphocytic Mg concentration remained remarkably stable. CONCLUSIONS: Our data does not support the conclusion that Mg deficiency is the primary cause of pre-eclampsia.