Introduction: Hypomagnesemia is a common but less frequently monitored electrolyte abnormality in hospitalized patients, especially in the critically ill. Accumulating evidence suggests a potential association between magneasium levels and the morbidity and mortality of critically ill patients. Assessment of electrolytes upon admission to the ICU is necessary to identify patients at risk and to guide the appropriate management during ICU stay. Aims and Objectives: 1. To assess the levels of serum magnesium in critically ill patients on admission. 2. To evaluate its relationship with APACHE IV (Acute Physiology and Chronic Health Evaluation) score, ventilator support and its duration, length of stay, and mortality. Materials and Methods: 80 patients admitted to the Medical intensive care unit (MICU) were taken for the study. The subjects studied were monitored for serum magnesium levels on Day 1 of intensive care unit (ICU) admission and were followed to assess duration of ventilator support, length of ICU stay and mortality. Results: At admission, 45% patients had hypomagnesemia, 6% patients had hypermagnesemia and 49% patients had normomagnesemia. Patients with lower magnesium levels had higher need and longer duration of mechanical ventilation, more frequently had hypokalemia, hypocalcemia, hyponatremia and a higher mortality rate (39% vs 25%). APACHE IV score and length of stay did not significantly vary in hypomagnesemic patients. Conclusion: Since the presence of hypomagnesemia at admission in the ICU patients is associated with a worse prognosis, recognition and treatment of hypomagnesemia in patients entering the ICU are important.