BACKGROUND Previous studies on dietary magnesium and risk of ischemic heart disease (IHD) have yielded inconsistent results, in part because of a lack of direct measures of actual magnesium uptake. Urinary excretion of magnesium, an indicator of dietary magnesium uptake, might provide more consistent results. OBJECTIVE The objective was to investigate whether urinary magnesium excretion and plasma magnesium are associated with IHD risk. METHODS We examined 7664 adult participants free of known cardiovascular disease in the Prevention of Renal and Vascular End-Stage Disease (PREVEND) study-a prospective population-based cohort study. Urinary magnesium excretion was measured in 2 baseline 24-h urine collections. RESULTS Mean ± SD urinary magnesium excretion was 4.24 ± 1.65 mmol/24 h for men and 3.54 ± 1.40 mmol/24 h for women. During a median follow-up of 10.5 y (IQR: 9.9-10.8 y), 462 fatal and nonfatal IHD events occurred. After multivariable adjustment, urinary magnesium excretion had a nonlinear relation with IHD risk (P-curvature = 0.01). The lowest sex-specific quintile (men: <2.93 mmol/24 h; women: <2.45 mmol/24 h) had an increased risk of fatal and nonfatal IHD (multivariable HR: 1.60; 95% CI: 1.28, 2.00) compared with the upper 4 quintiles of urinary magnesium excretion. A similar increase in risk of the lowest quintile was observed for mortality related to IHD (HR: 1.70; 95% CI: 1.10, 2.61). No associations were observed between circulating magnesium and risk of IHD. CONCLUSIONS Low urinary magnesium excretion was independently associated with a higher risk of IHD incidence. An increased dietary intake of magnesium, particularly in those with the lowest urinary magnesium, could reduce the risk of IHD.