The association between bariatric surgery outcome and blood levels of fibroblast growth factor 21 (FGF21) remains controversial. Many patients displayed stable or decreased FGF21 one year after bariatric surgery. Nevertheless, there is often an early increase FGF21 concentration in the post-surgery period. The aim of this study was to investigate the relationship between 3-month FGF21 response and percentage total weight loss at one year after bariatric surgery. In this prospective monocentric study, a total of 144 patients with obesity grade 2–3 were included; 61