BACKGROUND: Low serum magnesium levels predict cardiovascular and all-cause mortality in patients with typ 2 diabetes. SETTING: Outpatient clinic of obesity and central hospital. OBJECTIVES: To assess long-term alterations in circulating magnesium status after Roux-en-Y gastric bypass (RYGB) surgery and associations with remission of type 2 diabetes (T2D). METHODS: Retrospective analysis of 5-year outcomes of plasma magnesium (p-Mg) and glucometabolic statuses in patients who underwent primary RYGB and who completed the annual follow-up program. Data were investigated from 84 patients without diabetes and 62 with T2D before RYGB, who showed either prolonged remission (n = 30), temporary remission (n = 16), or no remission (n = 16) after surgery. RESULTS: Body mass indexes before RYGB were similar in patients with and without T2D, irrespective of remission. The patients not achieving remission showed longer diabetes durations; higher circulating glucose levels; more intensive antidiabetic drug treatment, including insulin; and significantly lower p-Mg concentrations (.73 [±.08] mmol/L compared with .80-.82 [±.07] mmol/L, respectively; P < .01) than the groups showing remission or without diabetes before surgery. After RYGB, the p-Mg increased similarly, by 10-12% in the groups with T2D before surgery, irrespective of remission; however, the nonremission group did not reach the p-Mg levels registered in the other groups after follow-up. The nonremission group reached .82 (.09) mmol/L, compared with .87 (.06) and .88 (.08) mmol/L (P