To assess the hypothesis that magnesium intake is beneficial in the primary prevention of hypertension, 28,349 female United States health professionals aged > or =45 years participating in the Women's Health Study (WHS), who initially reported normal blood pressure (systolic blood pressure <140 mm Hg, diastolic blood pressure <90 mm Hg, no history of hypertension or antihypertensive medications), were prospectively studied. A semi-quantitative food frequency questionnaire was used to estimate magnesium intake. During a median follow-up of 9.8 years, 8,544 women developed incident hypertension. After adjustment for age and randomized treatment, magnesium intake was inversely associated with the risk for developing hypertension; women in the highest quintile (median 434 mg/day) had a decreased risk for hypertension (relative risk 0.87, 95% confidence interval [CI] 0.81 to 0.93, p for trend <0.0001) compared with those in the lowest quintile (median 256 mg/day). This inverse association was attenuated but remained significant after further adjustment for known risk factors. In the fully adjusted model, the relative risks were 1.00 (95% CI 0.95 to 1.10), 1.02 (95% CI 0.95 to 1.10), 0.96 (95% CI 0.89 to 1.03), and 0.93 (95% CI 0.86 to 1.02) (p for trend = 0.03). Similar associations were observed for women who never smoked and reported no history of high cholesterol or diabetes at baseline. In conclusion, the results suggest that higher intake of dietary magnesium may have a modest effect on the development of hypertension in women.