Calcium and blood pressure. An epidemiologic perspective. We reviewed the research literature on the epidemiologic relationship between blood pressure levels and calcium, with an emphasis on dietary intake. A conceptual framework for causal inference is summarized; then the designs and results of observational and intervention studies are presented. Of 25 reports of observational studies relating intake of calcium or calcium-rich foods to blood pressure, the majority found some evidence of an inverse association. However, many analyses did not support this relationship, and only two studies have confirmed the inverse association with a prospective design. Nineteen randomized controlled clinical trials of calcium supplementation have been reported, excluding those exclusively in pregnant women. Eleven of these showed no significant effects on blood pressure; in two trials, both systolic and diastolic pressure were significantly reduced; and in the remainder results were equivocal. Pooled analyses yielded estimates of a small (1.8 mm Hg), significant reduction in systolic blood pressure, but no effect on diastolic pressure. Epidemiologic relationships with serum and urinary calcium, and the possible mechanisms of these effects, are also discussed. We conclude that the evidence from studies in humans is suggestive, but not conclusive, regarding a role for calcium in hypertension. Recommendations for further epidemiologic studies are presented. 