100 patients were followed for 6-9 years after Roux-en-Y gastric bypass for morbid obesity. They were randomly selected from 600 patients operated on between 1980-85. Their initial weight averaged 118 +/- 16 kg, which was 196 +/- 25% of their ideal body weight. They reached their lowest weight during an average of 15.5 months after surgery, at which time 37% were at their ideal weight, 42% were 110-130% of their ideal weight and 21% were 131-192% of their ideal weight. 90% had lost more than 50% of their excess weight. 24% had lost all their excess weight or even more, which could jeopardize nutritional status. Longer follow-up of 6-9 years showed weight gain, with 24% having become morbidly obese again, 74% having lost more than 50% of their excess weight and only 7% having lost all their excess weight. We conclude that Roux-en-Y gastric bypass induces long term weight loss and largely prevents return of morbid obesity, as long as 6-9 years after surgery.