A patient cannot consent to an operation without being adequately informed. There are a number of different operations in use today for treatment of severe obesity. The variations are designed to (1) limit food intake and/or (2) create malabsorption. The surgeon has a duty, according to the law of informed consent, to provide all of the information necessary for a reasonable person to decide whether to consent to the operation recommended. Changes in anatomy, function and risk therefore need to be explained. When only limitation of intake is planned, as in vertical banded gastroplasty (VBG), the patient should know how large the pouch will be and how the outlet will be stabilized. When both intake restriction and malabsorption are planned, as in Roux-en-Y gastric bypass (RGB), or biliopancreatic diversion (BPD), the patient should know whether there will be a larger pouch (less restriction) and a short common channel (more malabsorption) as in BPD or a smaller pouch and less malabsorption. Patients should know that if they have an operation that uses maximum malabsorption to bring weight to a nearly normal level, the risk of malnutrition will be increased, which may require further hospitalization and possible operative treatment. When the duodenum is to be bypassed, the patient should know that this will impair iron and calcium absorption, and that access to this area for radiologic and endoscopic procedures may not be possible. Simple drawings can be used to explain what is planned and how the operation will determine body weight, side-effects, and risk.