OBJECTIVES: Hyponatremia (HN) is the most common electrolyte abnormality in hospitalized patients. Because of the methodological difficulties, little is known regarding the impact and management of HN on patient outcomes and health care resource usage. The HN Registry is a novel prospective effort to document the clinical management and healthcare outcomes of HN. METHODS: After informed consent or waiver, data were extracted from medical charts of patients enrolled in the HN registry. HN was defined as a serum sodium ≤ 130 mmol/L.  Data from these eligible patients are summarized by sample size (n) and percentage (%) for categorical data, and mean and standard deviation for continuous data. Since there is no universal definition, length of stay (LOS) was calculated in several ways including LOS from date of HN identification (LOS 1), LOS from date of HN treatment initiation (LOS 2), and LOS limited to cases where treatment started within 2 days of HN identification (LOS 3). RESULTS: A total of 3795 of the 4909 patients enrolled at 253 (US=160, EU=93) sites between Sept 2010 and January 2013 had sufficient data for analysis.  For fluid restriction, LOS 1=8.1±8.4, LOS 2=7.4±8.9, LOS 3=7.4±9.0. For pharmacological therapy, LOS 1=8.7±8.9, LOS 2=7.1±8.2, LOS 7.4±6.9.  For hypertonic saline, LOS 1=7.0±7.3, LOS 2=6.1±6.0, LOS 3=3.7±2.2, LOS 4=6.3±6.3. CONCLUSIONS: Our analysis shows a considerable of LOS variability by HN management and LOS definition.  By correcting the analysis for baseline factors and outliers, the effect of HN management on LOS may become clearer.