Introduction. Hyponatremia is a frequently observed electrolyte disorder among patients with cancer. In 1957, Schwartz et al. reported the first case of a patient with hyponatremia due to SIADH, secondary to lung cancer. From that moment on, there has been data published that indicates patients with SIADH are less responsive to chemotherapy, have greater predisposition to central nervous system metastases and are often characterized by an advanced stage of cancer during time of diagnosis. Hyponatremia has many possible causes, and the differential diagnosis can pose a challenge. Aim. The aim of the study was to consider the occurrence of secondary hyponatremia in the course of cancer and the significance of this disorder in the prognosis of the disease. Methods. An analysis of the clinical history of the patient and a review of available literature. Results. A 66-year old patient with hyponatremia was admitted to the Department of Endocrinology, and lung cancer was determined as the cause of the aforementioned electrolyte disorder. Conclusion. SIADH secondary to cancers should be included in a differential diagnosis of every case of hyponatremia of undetermined etiology.