OBJECTIVES We examined the probability of an obese person attaining normal body weight. METHODS We drew a sample of individuals aged 20 years and older from the United Kingdom's Clinical Practice Research Datalink from 2004 to 2014. We analyzed data for 76,704 obese men and 99,791 obese women. We excluded participants who received bariatric surgery. We estimated the probability of attaining normal weight or 5% reduction in body weight. RESULTS During a maximum of 9 years' follow-up, 1283 men and 2245 women attained normal body weight. In simple obesity (body mass index = 30.0-34.9 kg/m(2)), the annual probability of attaining normal weight was 1 in 210 for men and 1 in 124 for women, increasing to 1 in 1290 for men and 1 in 677 for women with morbid obesity (body mass index = 40.0-44.9 kg/m(2)). The annual probability of achieving a 5% weight reduction was 1 in 8 for men and 1 in 7 for women with morbid obesity. CONCLUSIONS The probability of attaining normal weight or maintaining weight loss is low. Obesity treatment frameworks grounded in community-based weight management programs may be ineffective.