Hyponatremia has been associated with excess long-term morbidity and mortality. However, effects during hospitalization are poorly studied.To examine the association of hyponatremia with the risk of in-hospital mortality, 30-day readmission, and other short-term adverse events among medical inpatients.Population-based cohort study using a Swiss claims database from January 2012 to December 2017.Medical inpatients.Hyponatremic patients were 1:1 propensity-score matched with normonatremic medical inpatients.The primary outcome was a composite of all-cause in-hospital mortality and 30-day hospital readmission. Secondary outcomes were intensive care unit (ICU) admission, intubation rate, length-of-hospital stay (LOS), and patient disposition after discharge.After matching, 94`352 patients were included in the cohort. Among 47`176 patients with hyponatremia, 8`383 (17.8%) reached the primary outcome compared with 7`994 (17.0%) in the matched control group (OR 1.06 [95%CI, 1.02 to 1.10], p=0.001). Hyponatremic patients were more likely to be admitted to ICU (OR 1.43 [95%CI, 1.37 to 1.50], p&0.001), faced a 56% increase in prolonged LOS (95%CI 1.52 to 1.60, p&0.001), and were admitted more often to a post-acute care facility (OR 1.38 [95%CI 1.34 to 1.42, p&0.001). Of note, patients with the syndrome of inappropriate antidiuresis (SIAD) had lower in-hospital mortality (OR 0.67 [95%CI, 0.56 to 0.80], p&0.001) as compared with matched normonatremic controls.In this study, hyponatremia was associated with increased risk of short-term adverse events, primarily driven by higher readmission rates, which was consistent among all outcomes except for decreased in-hospital mortality in SIAD patients.