Magnesium (Mg) is critically involved in the pathophysiology of multiple human diseases; nevertheless, Mg disorders are often poorly considered in the clinical practice. To update the prevalence and incidence of hypomagnesemia and hypermagnesemia in a real-life scenario, which better represents clinical practice, we analyzed data from 12,696 patients whose Mg serum levels were measured from January 1, 2015, through December 31, 2017 at our University Hospital. Hypomagnesemia and hypermagnesemia were defined by Mg concentrations 3.8 mg/dL (1.5 mmol/L), in accordance with the reference values for magnesemia of our laboratory (1.5-3.8 mg/dL). The prevalence of hypomagnesemia and hypermagnesemia was 8.43% (n=1071) and 1.78% (n=226), respectively. Hypomagnesemia occurred more frequently in females compared with males [53.3% (n=560) versus 47.7% (n=511), χ=4.03, p<0.045]; the highest prevalence of hypomagnesemia was found in patients over 65 yr. [59.01% (n=632)], when compared with the other age groups [59.01% (n=632) versus 9.52% (n=102) in patients aged 0-18 yr. and 31.46% (n=337) in patients between 19 and 65 yr., χ=592.64; p<0.0001)]. Incidence of hypomagnesemia decreased over time in subjects over 65 yr. (r=-0.99; p=0.07). Geriatrics, oncology, and intensive care division showed the highest incidences of hypomagnesemia. Mg disorders and remarkably hypomagnesemia are quite common in the clinical practice, particularly in older hospitalized patients. Thus, they should be routinely checked and corrected.