Introduction Obese kidney allograft recipients have an increased risk of surgical complications, delayed graft function(DGF), prolonged hospital stay and late graft failure. However, there is lack of information regarding the effect of body mass index(BMI) variation after kidney transplantation(KT). Methods and Materials In this longitudinal study, we used data from Catalan Renal Registry including first KT recipients within 1990 and 2011. The annual change on post-transplantation BMI was calculated all patient follow-up (until December 2015). Main outcome variables were DGF, eGFR(CKD-EPI), patient and graft survival. Statistical analysis was adjusted for variables impacting on outcome. Results A total of 5,983 kidney trasnplant recipients were included. Obesity was observed in 609 patients(10.9%) at the time of transplantation. Obese patients were transplanted more recently, were younger and received kidneys from younger donors. Incidence of DGF was significantly higher in obese (40.38% vs 29.5%, P7% was associated with worse graft survival. Conclusion Our conclusion is that BMI reduction after KT was not associated with eGFR improvement and only in those with a reduction of >7% patient survival was better with worse long-term graft survival.