Hypomagnesaemia is a common finding that occurs in hospitalised patients and can be caused by renal or extrarenal wasting. Because it is not routinely tested, even severe hypomagnesaemia can be missed. In cases of extrarenal wasting, the kidney can decrease its magnesium excretion to very low levels. A systematic diagnostic approach allows the proper diagnosis and treatment of hypomagnesaemia. We report on a 72-year-old Caucasian male patient with repeated drops and amyosthenia caused by severe hypomagnesaemia. In this report, we discuss the pathophysiology, clinical signs, diagnostic investigation and treatment of magnesium depletion.