BACKGROUND Hyponatremia is a common and potentially life threatening problem in patients with neuroendocrine tumors. METHODS A 54-year-old female with a history of smoking and narcotic dependency presented to her primary care physician with nausea, vomiting, fatigue, malaise, ataxia and a serum sodium of 100 mEq/L. A chest computerized tomography (CT) revealed a 4.1 x 4.9 cm precarinal/pretracheal mass encircling the right brachiocephalic vein. A mediastinal biopsy found a malignant, intermediate-size small cell (oat cell) carcinoma. Saline infusion along with intravenous furosemide successfully corrected her hyponatremia. Unfortunately, the patient later died of complications related to her cancer and cancer therapy. CONCLUSIONS Paraneoplastic hyponatremia can be severe, but even severe cases may be successfully treated.