Biliopancreatic diversion (BPD) has made reacceptable the malabsorptive approach to the surgical treatment of obesity. The procedure, in a series of 2241 patients operated on during a 21-year period, caused a mean permanent reduction of about 75% of the initial excess weight. The indefinite weight maintenance appears to be due to the existence of a threshold absorption capacity for fat and starch, and thus energy, and the weight loss is partly due to increased resting energy expenditure. Beneficial effects other than those consequent to weight loss or reduced nutrient absorption included permanent normalization of serum glucose and cholesterol without any medication and on totally free diet in 100% of cases, both phenomena being due to a specific action of the operation. Operative mortality was less than 0.5%. Specific late complications included anemia, less than 5% with adequate iron or folate supplementation (or both); stomal ulcer, reduced to 3.2% by oral H2-blocker prophylaxis; bone demineralization, increasing up to the fourth year and tending to decrease thereafter, with need of calcium and vitamin D supplementation; neurologic complications, totally avoidable by prompt vitamin B administration to patients at risk; protein malnutrition, which was reduced to a minimum of 3% with 1.3% recurrence, in exchange with a smaller weight loss, by adapting the volume of the gastric remnant and the length of the alimentary limb to the patient's individual characteristics. It is concluded that the correct use of BPD, based on the knowledge of its mechanisms of action, can make the procedure an effective, safe one in all hands.