CONTEXT: Hyponatremia is the most common electrolyte disturbance in hospitalized patients. Known risk factors are heart or renal failure, excessive drinking, or the use of diuretics. The incidence of hyponatremia may also be influenced by climate. OBJECTIVE: Analyzing the influence of outdoor temperature and relative humidity on the incidence and etiology of hyponatremia. DESIGN: Cohort A: cross-sectional study from January 2011 to December 2016. Cohort B: prospective observational study from June 2011 to August 2013. SETTING: Emergency departments of two tertiary centers. PATIENTS: Cohort A: patients with plasma sodium