Objective: To determine the frequency and associated risk factors of hypomagnesemia in pediatric intensive care unit on admission in a developing country. Methods: It is a retrospective chart review of 179 children aged 1 mo - 15yr admitted in Pediatric Intensive Care Unit of our university during 18 months and recorded serum Mg level on admission. Patients were divided into two groups according to their Mg level (Normo-magnesemic and Hypomagnesemic) and their p-value, crude and adjusted odds ratios (AoR) were calculated. Results: Upon admission in PICU 79(44%) patients were found hypomagnesemia. There was no difference in age and gender between two groups. The important risk factors identified were age greater than one yr (p 0.05, AOR 3.71), sepsis (p 0.03, AOR 3.11), hypokalemia (p 0.06, AOR 1.8), hypocalcemia (p 0.05, AOR 1.6), diuretic use (p 0.05, AOR 1.37), Aminoglycoside use (p 0.003, AOR 3.12), and hospitalization greater than five days (p 0.03, AOR 1.71). Those with normomagnesemic had higher mortality rate (32/100 or 32%) than those with hypomagnesemia (22/79 or 27.8%). Conclusion: The present finding indicates that hypomagnesemia is a common among PICU patients and is influenced by several factors.