Background: Hyponatremia is a commonly occurring electrolyte disturbance among hospitalized patients, and many studies suggest that individual serum sodium levels may have a prognostic role in assessing probability of mortality. Aim of the study: Determination of hyponatremia prevalence in patients admitted to the Oncology Teaching Hospital. Method: A six-month cross-sectional study of hyponatremia prevalence was conducted in the Oncology ward in Medical City. The patients (105) who were admitted for more than three days with different types of malignancies and causes of admission were investigated for serum sodium, blood urea, random blood sugar, and urinary sodium (single measurement). These results were compared to the sodium levels of control of non-cancer individuals (135) selected using the same exclusion criteria. Results: The prevalence of hyponatremia in the admitted patients was 43.7%, whereas it was 0.73% in the control group. Conclusion: The prevalence of hyponatremia in Oncology Teaching Hospital, Medical City ward, was higher than that reported in previous studies (conducted in international cancer centers); hypoosmolar hyponatremia prevalence is almost similar to that stated in previous studies.