Hyponatremia is the most common hydroelectrolyte disorder in hospita-lized patients. It is unclear whether there are differences between severe hyponatremia (<125 mEq/L) and very severe hyponatremia (<115 mEq/L) in terms of etiology, response to therapy, and mortality. Aim: Describe the etiology, symptoms, response to treatment and mortality of hospitalized adults with severe and very severe hyponatremia. Methods: Retrospective study reviewing clinical and laboratory records of hospitalized patients between 2010 and 2020. Results: records from 398 patients were ob-tained. The average age was 71,4 years. The average sodium Na+ upon admission was 116,5 mEq/L. 69,1% corresponded to severe hyponatremia and 30,9% to very severe. In the severe group, there were more patients with cirrhosis and renal failure, while in the very severe group, there was more use of SSRIs, hypokalemia, and hypouricemia, and more therapy with hypertonic sodium. 14,1% had no response (Na+<130), 49,0% had a partial response (Na+130-134), and 36,9% had a total response (Na+>= 135). Those without response had more edema and cirrhosis and lower urea nitrogen and creatinine values, while those with response had more use of thiazides, ARA II, renal failure, as well as therapy with isotonic saline solution. Overall mortality was 10,1%. Survivors had more nausea and vomiting, used thiazides and ARBs on admission, and received more isotonic and hypertonic saline solutions. The deceased received more albumin and isotonic saline solutions, responded more to therapy, and cirrhosis was more common. Conclusions: Differences related to etiology, response to treatment, and mortality were observed between patients with severe and very severe hyponatremia. These fin-dings may generate hypotheses for prospective studies of hyponatremia in hospitalized patients.