BACKGROUNDMagnesium is the fourth most abundant cation in the body. Hypomagnesaemia is common in critically ill patients, and there is strong, consistent clinical evidence that it is associated with increased need for mechanical ventilation, prolonged ICU stay and increased mortality. There is a current need to expand our knowledge with regards to magnesium homeostasis and its impact on survival and prognosis of critically ill patients. Aims and Objectives-To study the serum total magnesium in critically ill patients and to assess the primary critical medical conditions associated with abnormalities of serum magnesium and to correlate serum magnesium levels with patient outcome in terms of length of stay and mortality.