This study assessed the hypothesis that greater magnesium intake is associated with reduced risk for cardiovascular disease (CVD), including myocardial infarction (MI) and stroke, in a large prospective cohort of women. In 1993, a semi-quantitative food frequency questionnaire was used to assess magnesium intake in 39,876 female health professionals aged 39 to 89 years who had no history of CVD or cancer. During a median of 10 years of follow-up, 1,037 incident cases of CVD were identified, including 280 nonfatal MIs and 368 strokes. After adjustment for age and randomized treatment status, magnesium intake was not significantly associated with risk for incident CVD. Comparing the highest quintile of magnesium intake (median 433 mg/day) with the lowest quintile (median 255 mg/day), the relative risks were 0.87 (95% confidence interval [CI] 0.72 to 1.05, p for trend = 0.24) for total CVD, 0.88 (95% CI 0.70 to 1.12, p for trend = 0.34) for coronary heart disease (CHD), 1.03 (95% CI 0.72 to 1.49, p for trend = 0.96) for nonfatal MI, 1.11 (95% CI 0.61 to 2.00, p for trend = 0.95) for CVD death, and 0.87 (95% CI 0.64 to 1.18, p for trend = 0.55) for total stroke. Additional adjustment for other CVD risk factors did not materially change the observed null associations. In conclusion, the results do not support the hypothesis that magnesium intake reduces the development of CHD, although a modest inverse association with stroke cannot be ruled out.