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Drinking water (DW) is an important dietary source of magnesium. Israel has recently increased desalinated seawater (DSW) production for DW, but negligible magnesium content in DSW may pose a risk of hypomagnesemia and consequential adverse cardiovascular effects. Consecutive acute myocardial infarction (AMI) patients ( n = 380, age 35-75 years), hospitalized in 2015-2017 with ST-segment elevation myocardial infarction (STEMI), were divided into two groups based on their domicile region having a major supply of DSW ( n = 250, 65%) or not (non-DSW; n = 130, 35%). We evaluated admission serum magnesium concentrations in patients, magnesium levels in tap water, 1-year all-cause mortality, and major adverse cardiovascular events (MACE), including all-cause mortality, nonfatal myocardial infarction, rehospitalization for heart failure or angina pectoris, stroke, coronary artery bypass grafting, and percutaneous coronary interventions. Multivariate analyses were adjusted for age and sex. Serum magnesium concentrations (mean ± SD) were significantly higher among patients in the non-DSW group compared with the DSW group (1.95 ± 0.20 mg/dL and 1.81 ± 0.20 mg/dL, P < 0.001; respectively). Additionally, the mean residential DW magnesium level in the DSW group was 5.4 ± 2.2 mg/L compared with 25.1 ± 3.4 mg/L, P < 0.01 in the non-DSW group. Fewer patients (although not statistically significant) in the non-DSW group experienced major adverse cardiovascular events (MACE) or 1-year-all-cause mortality compared with the patients in the DSW group (12.4% and 20%, P = 0.065; respectively). In conclusion, in post AMI patients, we found nonsignificant higher MACE and 1-year mortality with the use of DSW.