The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is defined by euvolemic hyponatremia due to an inappropriate retention of free water under the effect of antidiuretic hormone. It is underdiagnosed despite well-defined criteria. Diagnosis involves a precise evaluation of volemia and the elimination of differential diagnoses. The etiologies are classified into four main groups : tumors, drugs, diseases of the central nervous system and lung diseases. In the case of a newly diagnosed SIADH, investigations depend on the clinical context and should at least include a chest radiograph or a chest CT-scan. Fluid restriction is the recommended first-line treatment for mild or moderate hyponatremia. However, only the etiologic treatment leads to the disappearance of SIADH.