The authors present the case of a 51-year-old female with a syndrome of inappropriate secretion of antidiuretic hormone (SIADH) due to a malignant thymoma. Laboratory examinations of the patient showed hyponatremia, plasma hypoosmolarity in the presence of concentrated urine and normal urine excretion of sodium. CT revealed a mass lesion in the mediastinum. A biopsy of the mediastinal mass was performed and the diagnosis of thymoma with SIADH was established. This is a rare description of a malignant thymoma associated with SIADH.