Background and Aim: Magnesium monitoring among critically ill patients have prognostic and therapeutic implications as they are predisposed to symptomatic and asymptomatic magnesium deficiency which may lead to the development of neurotoxicity, psychiatric problems and leading to increased morbidity and mortality. Hence the aim of the present study was to study the serum magnesium levels in critically ill patients. Materials & Method: Total of 120 cases was included in the study. They were divided into two groups: cases were 60 and controls were 60. The serum magnesium of the included patients was estimated with Colorimetry method. Results: Among the 60 cases, it was found that 45 patients were males and the remaining 15 were females. Among the 60 controls, 45 patients were males and 15 were females. Among the cases, 18 patients had diabetes mellitus whereas 12 patients out of 60 controls had association of diabetes mellitus. In our present study, the mortality rate was higher in cases i.e 30% as compared to 2% in controls. Conclusion: Hypomagnesemia is common in hospitalized patients, especially the critically ill. It has a higher morbidity and mortality among these patients. The assessment of serum magnesium concentration is inexpensive and easy to employ and provides important information about magnesium status in patients. Hypomagnesemia, when detected, may require correction for the management of those with critical illness for better outcomes.