Context: Hyponatremia is a common electrolyte disturbance in critically ill hence understanding its implications is important. Aims: This study was carried out to ascertain frequency, predisposing conditions and outcome in critically ill patients with hyponatremia on intensive care unit (ICU) admission. Settings and Design: This was an observational, prospective study of a series of ICU patients during a 12-month period. Materials and Methods: The patients were divided into two groups: Hyponatremic (serum sodium < 135 mmol/L) and Eunatremic groups (135-145 mmol/L). Clinical examination included volume status and drug history, biochemistries, clinical diagnosis and cause of hyponatremia. Statistical Analysis Used: Fisher′s exact test, unpaired t-tests Wilcoxon ranksum tests, profile-likelihood method, log-rank test and Kaplan-Meier curves were used. P < 0.05 were considered to be statistically significant. Results: In the hyponatremic group, the frequency of hyponatremia on ICU admission was 34.3%, most were euvolumic, 58.96%. Females comprised of 36.5%. The mean age was 60.4 ± 17.2. The Syndrome of inappropriate Antidiuretic Hormone (SIADH) criteria was met in ninety-one patients (36.25%), peumonia being the leading cause of SIADH. Patients with severe sepsis, elective surgery patients, renal failure and heart failure, cirrhosis of liver and subarachnoid hemorrhage were other more likely etiologic causes (P < 0.05). The hyponatremic group spent a longer time in the ICU (P = 0.02), had longer mechanical ventilator days (P < 0.05) and had an increased mortality rate (P = 0.01). Conclusions: Hyponatremia present on admission to the ICU is independent risk factors for poor prognosis.