Observation:Discussion:SIADH is characterized by a hypovolemic hyponatraemia, and must be evoked if the following criteria are present : plasmatic osmolarity <280 mOsm/kg or natraemia<134 mmol/L, urinary osmolality>100 mOsm/kg, clinical normal volemia, sodium urine concentration>40 mmol/L with normal fluid and sodium intake, exclusion of hypothyroidism, adrenal insufficency or diuretics intake. Additional criteria are low uricemia, low uremia, and normal creatinine, potassium and alkali reserve (1). In the face of a SIADH, the clinician can use few therapeutic strategies. Aetiological treatment and fluid limitation constitute the first line treatment. However, there are some situations where fluid limitation is not effective, and a pharmacological agent has to be introduced.A 78 year-old woman was referred for cough, chest pain, shortness of breath and tiredness. Her past medical history was composed of high blood pressure and dyslipidemia. Blood tests revealed hyponatraemia, and computed tomography analysis led to the diagnosis of a right lung adenocarcinoma with T2N1bM1 staging , that could not be surgically removed, as metastases were present ( fig.1).As dyspnea and tiredness increased, a chimiotherapy with pemetrexed was initiated. However hyponatraemia was still observed.