Abstract The syndrome of inappropriate secretion of antidiuretic hormone (ADH) was recognized in a 68-year-old man with a poorly differentiated metastatic adenocarcinoma of the prostate. Elevated levels of ADH were found in the tissues of the primary tumor and lymph node metastasis. The patient's clinical course is detailed and the pathophysiology of this syndrome is discussed. To our knowledge, this case is the ninth reported case of syndrome of inappropriate secretion of ADH with adenocarcinoma of the prostate. Antidiuretic hormone activity was proven in only three cases including this case.