We present a case of villous adenoma of the rectum, associated with severe fluid imbalance in a 69 year-old, previously healthy woman. Severe dehydration with hyponatraemia, hypokalaemia, acidosis/alkalosis and watery diarrhoea is typical. Clinical presentation may include fatigue, confusion, convulsions or coma. Intensive rehydration is the primary intervention, and causal treatment is always surgery. Normally full recovery is achieved.