Incidence

United States
Hyponatremia is the most common electrolyte disorder, with a marked increase among hospitalized and nursing home patients. A 1985 prospective study of inpatients in a US acute care hospital found an overall incidence of approximately 1% and a prevalence of approximately 2.5%. On the surgical ward, approximately 4.4% of postoperative patients developed hyponatremia within 1 week of surgery. Hyponatremia has also been observed in approximately 30% of patients treated in the intensive care unit.

International
Though clearly not indicative of the overall prevalence internationally, hyponatremia has been observed in as high as 42.6% of patients in a large acute care hospital in Singapore and in 30% of patients hospitalized in an acute care setting in Rotterdam.

-Hyponatremia is the most common electrolyte disorder in clinical practice. It is estimated that the incidence of hyponatremia in hospitalized patients in the U.S. is greater than one million. Hyponatremia is recognized as an independent contributor to negative patient outcomes in many chronic diseases, most notably CHF, as well as cirrhosis and SIADH.

References

http://emedicine.medscape.com/article/767624-overview
http://www.healthscout.com/ency/1/000394.html
http://au.biz.yahoo.com/070702/43/1arei.html