Background: Intensive care unit (ICU)-aquired dysnatremia is the disturbance of sodium level in patients whose sodium concentration was normal at ICU admission. Traditionally, researches about ICU-acquired dysnatremia have focused on hypernatremia, but ICU-acquired hyponatremia (IAH) has been frequently overlooked. We aimed to investigate the etiology, prognosis, and risk factors of IAH in medical ICU patients. Method: In this study, IAH was defined as hyponatremia (Na < 135mmol/L) developed within 48 hours after ICU admission. We retrospectively reviewed medical records of patients discharged from 2015 to 2019, and compared patients with IAH and normonatremia. Patients with abnormal sodium level on admission or dependence on renal replacement therapy were excluded. Primary outcome was ICU mortality. Result: Of 1409 patients, 265 (18.8%) patients had IAH. IAH patients were more severe than normonatremia patients in terms of sequential organ failure assessment (SOFA) and simplified acute physiology score 3 (SAPS 3) (8.8±4.22 vs 7.95±4.18, p=0.004; 56.36±17.41 vs 53.33±16.65, p=0.003)). Reasons for admission and underlying conditions were similar, but endocrine disorder (1.5% vs 0.2%, p=0.013) and underlying chronic kidney disease (1.9% vs 0.4%, p=0.025) were more common in IAH group. Baseline sodium level and renal function were significantly lower in IAH group (Na, 137.1±2.2mmol/ L vs 139.0±2.7mmol/L; BUN, 31.7±23.2mg/dl vs 28.6±21.1mg/dl; Cr, 1.6±1.5mg/dl vs 1.4±1.5mg/dl; glomerular filtration rate, 72.8±50.7ml/min/1.73m2 vs 82.3±67.97ml/min/1.73m2). SAPS3, endocrine disorder, and baseline sodium and potassium levels were associated with the development of IAH in multivariate logistic regression (1.015[1.006-1.024], p=0.001; 12.054[2.048-70.394], p=0.006; 0.735[0.689-0.784], p<0.001; 1.372[1.162-1.620], p<0.001, odds ratio [95% confidence interval], p-value, respectively). ICU mortality were not different between the both groups (17.4% vs 16.1%, p=0.645). However, IAH group showed trend of increased ICU length of stay (10.1±12.7 days vs 8.5±10.7 days, p=0.097). Conclusion: IAH was not an independent risk factor of ICU mortality, but a maker of disease severity and impaired renal function.