The weights of 207 morbidly obese patients were reduced via prolonged fasting. Half the patients fasted for close to two months, losing a mean of 28.2 kg; one fourth fasted for less than one month; and the other fourth fasted for more than two months, with a mean 41.4-kg loss. This latter group was heavier initially, and more than 50% attained near-normal weight. Patients with onset of obesity in childhood had the lowest tolerance for fasting and the lowest success rate in attaining normal weight. Over a 7.3-year follow-up period in 121 patients, the reduced weight was maintained for the first 12 to 18 months. Subsequently, regain proceeded equally in all groups irrespective of length of fast, extent of weight loss, or age at onset of obesity. Regain to original weight occurred in 50% within two to three years and only seven patients remained at their reduced weights. Regain to greater than original weight was more common in childhood-onset obesity.