Background/Aim: Cardiovascular disease is the most serious cause of death in patients on hemodialysis. Low serum magnesium (Mg) and high serum calcium (Ca) levels have been associated with poor outcome and cardiovascular mortality in patients on maintenance and initiation dialysis. As a more accurate marker is warranted, we evaluated the efficacy of a novel serum Ca-Mg marker of all-cause and cardiovascular mortality that indicates vessel calcification. Methods: We recruited 378 consecutive patients with end-stage renal disease who started dialysis between January 2009 and December 2015 at the Japanese Red Cross Ishinomaki Hospital. We collected data of patients' demographic characteristics and comorbidities from their electronic medical records. We retrospectively examined the association of the serum Ca-Mg ratio with all-cause and cardiovascular mortality using the Cox proportional hazard model, and determined the value that predicted cardiovascular death using the receiver operating characteristic (ROC) curve. Results: Overall, 253 patients with serum Mg and Ca data were analyzed. The 3-year survival rate of this group was 0.70 (95% confidence interval [CI] 0.55-0.80), and the hazard ratio for the risk of death was 3.94 (95% CI 1.37-11.31). The 3-year cardiovascular mortality rate was 0.12 (95% CI 0.05-0.23), which was significantly higher than that of the other groups. The ROC curve of cardiovascular mortality with the Ca-Mg ratio was greater than that of Mg (area under the curve 0.75 vs. 0.69, p = 0.037). Conclusion: A high Ca-Mg ratio was significantly associated with all-cause and cardiovascular mortality, and it was more accurate than serum Mg.