INTRODUCTION: Hyponatremia is the most common electrolyte disorder detected by bio-chemical laboratories in the present time and interest about it increases because of new informations about its consequences. AIM: Objective of our work was to determine the prevalence of hyponatremia in patients hospitalized at clinic of internal medicine and to analyze its most common causes. PATIENTS AND RESULTS: The total number of hospitalized patients for a period of 6 months was 1,203. Severe hyponatremia, i.e. S Na < 130 mmol/ l was detected in 83 cases, i.e. 6.9% of all patients. 14 patients, i.e. 1.15% have more severe hyponatremia with S Na < 120 mmol/ l. The most common type of hyponatremia was hypervolemic hyponatremia (n = 41; 49.4%) associated with liver cirrhosis and hearth failure, less common was euvolemic hyponatremia (n = 28; 33.7%) and least common was hypovolemic hyponatremia (n = 14; 16.9%). The most common cause of euvolemic hyponatremia was syndrome of inappropriate antidiuretic hormone secretion (SIADH) and thiazide diure-tics therapy. Next to thiazide diuretics, other common cause of euvolemic hyponatremia, and so SIADH, was therapy by selective serotonin reuptake inhibitors (SSRI). CONCLUSION: The authors point out, that it is necessary to control serum sodium concentration on regular base in case of longterm therapy by thiazide diuretics or SSRI, especially in elderly patients.