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Mason performed the first gastric bypass (GBP) for obesity in 1967 after having observed substantial weight loss in patients operated for gastric ulcer.  The weight loss after GBP is 30 % and 2/3 of patients with type II diabetes can stop their medication. Half of the patients can stop medication for hypertension or hyperlipidemia, 75% are cured from reflux and obstructive sleep apnea. GBP prolongs survival especially among diabetics. Restriction and malabsorption is of minor importance for weight loss. The mechanisms underlying weight loss and diabetes remissions are endocrine. GBP enhances food passage to the distal small bowel where the food contact stimulates release of satiety hormones from endocrine cells. Elevated levels of these hormones cause weight loss, increases insulin secretion and counteracts weight induced decrease in resting metabolic rate, an important starving protection mechanism that activates when fasting and makes traditional weight loss difficult.