OBJECTIVE: To explore the long-term effect of calcium supplementation during pregnancy on the offspring's blood pressure during childhood. DESIGN: Follow up of a population enrolled in a double blind, randomised, placebo controlled trial. SETTING: Perinatal research unit, World Health Organisation's collaborative research centre. SUBJECTS: 591 children at a mean age of 7 years whose mothers were randomly assigned during pregnancy to receive 2 g/day of elemental calcium (n = 298) or placebo (n = 293). MAIN OUTCOME MEASURES: Mean blood pressure and rate of high blood pressure of children. RESULTS: Overall, systolic blood pressure was lower in the calcium group (mean difference -1.4 mm Hg; 95% confidence interval -3.2 to 0.5) than in the placebo group. The effect was found predominantly among children whose body mass index at assessment was above the median for this population (mean difference in systolic blood pressure -5.8 mm Hg (-9.8 mm Hg to -1.7 mm Hg) for children with an index > 17.5 and -3.2 mm Hg (-6.3 mm Hg to -0.1 mm Hg) for those with an index of > 15.7 to 17.5). The risk of high systolic blood pressure was also lower in the calcium group than in the placebo group (relative risk 0.59; 0.39 to 0.90) and particularly among children in the highest fourth of body mass index (0.43; 0.26 to 0.71). CONCLUSION: Calcium supplementation during pregnancy is associated with lower systolic blood pressure in the offspring, particularly among overweight children.