We report on a 75-year-old man with a history of metastatic prostate cancer who presented with haematuria, peripheral oedema, metabolic alkalosis, hypokalaemia, and hypertension. Laboratory evaluation was compatible with the diagnosis of adrenocorticotropic hormone (ACTH)-dependent cushing's syndrome and suggestive of ectopic ACTH production. Pathology of a prostate biopsy specimen showed a large cell neuroendocrine carcinoma (LCNEC) of the prostate. This report describes a case of Cushing's syndrome that was probably caused by ectopic ACTH secretion by a LCNEC of the prostate.