McKittrick-Wheelock syndrome is characterized by extracellular volume depletion and severe fluid and electrolyte imbalance caused by secretory tumors of the rectum, usually villous adenomas. Patients usually present signs of dehydration, hyponatremia, hypokalemia, and prerenal azotemia due to the chronic diarrhea, rich in ions and water. Since this is a potencially lethal entity without treatment, and a very unusual condition, we report a clinical case. Treatment consists in surgical resection of the tumor, although endoscopy polypectomy has also been reported.