Objectives: To evaluate the association between magnesium levels, morbidity and mortality in critically ill elderly patients admitted in ICU. Methods: A cross-sectional study was done on patients admitted to the intensive care unit (ICU) of the Geriatrics department at a University Hospital over 1.5 years period. Data collected included patients demographics and medical history, length of stay in the ICU. Lab tests included serum Magnesium on admission, serum sodium and potassium. In addition, the Acute Physiology and Chronic Health Evaluation (APACHE) II score and the Sequential Organ Failure Assessment (SOFA) score were determined at the day of admission. Results: A single center, cross sectional study was done on 100 patients who were admitted to the Geriatrics medical ICU at Ain Shams university Hospital in Cairo. At admission 23% of patients had low magnesium (Mg) level and 59% had normal magnesium level and 18% had high Mg level. The mean age of patients with hypomagnesaemia was 71.69 years old while those with normal magnesium level were 69.5 years old and that with hypermagnesaemia were 69.7 years old. The length of ICU stay was longer among patients with high Mg level (11.7 ± 14.4 days) and among those with normal magnesium level (7.6 ± 5.8 days) versus (7.5 ± 3.8 days) those with low Mg level, among the studied groups (55.6%) of patients with Hypermagnesaemia died versus (44.1%) of patients with normal Mg level. Among the studied groups, those with high Mg level were found to have higher APACHE II score (20.7 ± 7.4) and SOFA score (5.5 ± 3.6) than the other subjects. Significant positive correlation was found between mean Mg level and SOFA score. Conclusion: Development of hypermagnesaemia during an ICU stay is associated with bad prognosis. Monitoring of serum magnesium levels may have prognostic, and perhaps therapeutic, implications.