BACKGROUND Hypomagnesaemia is associated with other electrolyte abnormalities like hypokalaemia, hyponatremia, and hypophosphatemia. We wanted to study the serum magnesium levels in critically ill patients, and correlate the serum magnesium levels with patient outcome and other parameters like duration of stay in ICU, ventilator support and APACHE-II (Acute Physiology and Chronic Health Evaluation-II) score. METHODS The study included all the cases admitted in the ICU of Narayan Medical College & Hospital, with variable medical conditions within 6 months fulfilling the inclusion criteria. Demographic data (age and sex), medical history, surgical history, medications administrated and length of ICU stay were recorded for each patient. The severity scoring system used was Acute Physiology and Chronic Health Evaluation-II (APACHE-II). RESULTS Prevalence of Hypomagnesaemia in the present study was 60.2 %. Mortality and mechanical ventilator support (2.7 % and 28.4 %) in normomagnesemia subjects were significantly lesser than hypomagnesaemia subjects (33.9 % and 54.5 % respectively). CONCLUSIONS Hypomagnesaemia is a common electrolyte imbalance in critically ill patients. It is associated with higher mortality and morbidity in critically ill patients and is also associated with more frequent and more prolonged ventilatory support. KEYWORDS Critically Ill, Hypomagnesaemia, APACHE-II Score, Mortality, Ventilator Support