Background: Low serum magnesium levels may cause fatal ventricular arrhythmias. However, their long-term effects on mortality and morbidity in chronic heart failure patients are relatively unknown. Methods: We studied 1569 chronic systolic and diastolic heart failure patients with normal sinus rhythm who participated in the Digitalis Investigation Group trial and had serum magnesium data available at one month. Of these, 741 patients had normal (> 2 mEq/L) and 828 had low (≤ 2 mEq/L) serum magnesium levels. Propensity scores for having low serum magnesium levels were calculated for each patient using a non-parsimonious multivariable logistic regression model, and were used to match 560 (76%) low-magnesium patients with 560 normal-magnesium patients. Effects of low-magnesium on mortality and hospitalization during a mean follow-up of 36 months were assessed using matched Cox regression analyses. Results: All-cause mortality occurred in 156 (rate, 915/10,000 person-years) normal- magnesium and 171 (rate, 1034/10,000 person-years) low-magnesium patients, respectively, during 1704 and 1653 years of follow-up (hazard ratio, 1.23; 95% confidence interval, 0.97-1.57; p = 0.089). Cardiovascular mortality occurred in 110 (rate, 646/10,000 person-years) normal-magnesium and 133 (rate, 805/10,000 person-years) low-magnesium patients (hazard ratio, 1.38, 95% confidence interval, 1.04-1.83, p = 0.024). Hazard ratios and 95% confidence intervals for all-cause and cardiovascular hospitalizations were respectively 1.18 (0.99-1.42; p = 0.068) and 1.14 (0.94-1.39; p = 0.182). Conclusions: In a propensity-matched population of ambulatory chronic heart failure patients who were balanced in all measured baseline covariates, serum magnesium level 2 mEq/L or less was associated with increased cardiovascular mortality, but had no association with cardiovascular hospitalization. © 2008 Elsevier Ireland Ltd. All rights reserved.