The intestinal bypass operation has come into wide use as a method for treating the grossly obese patient. Because it carries risks of morbidity and mortality, it is only suitable for those who meet certain minimal criteria. During the first year after surgery, weight loss varies from 9.1 to 68.0 kg [20 to 150 lb]. These patients have shown improved self-esteem and a reduction in the amount of depression. Mortality for this operation is approximately 4%; many serious complications (liver failure, pulmonary embolus, improper wound healing, severe loss of minerals, and vitamin deficiencies) occur. Hyperoxaluria may result from increased intestinal absorption. A dilated colon (pseudocolonic obstruction) may develop from bacterial overgrowth in the distal segment of the bypassed intestine. Since these serious complications affect 40% of the patients, the operation should be done by trained and skilled physicians who will also be able to manage the long-term postoperative period.