Magnesium is the second most abundant intracellular cation and the fourth most abundant cation in the body. Clinical manifestations of hypomagnesaemia include neuromusclar, neurological, psychiatric and cardiac arrhythmias including torsade de pointes resulting in sudden death. Incidence of hypomagnesaemia in hospitalised patients in common and there is a lack of clinical awareness. Clinicians should become familiar with the common conditions and therapeutics that are risk factors for underlying hypomagnesaemia and become familiar with magnesium replacement regimens. Two patients who suffered fatal complications in whom hypomagnesaemia was an important contributing factor are presented. Hypokalaemia and hypocalcaemia are common in severe magnesium deficiency and require concurrent monitoring and correcting.