*Purpose: Early post-kidney transplantation (KT) changes in physiology, medications, and health stressors likely impact body mass index (BMI). Hence, we sought to estimate trajectories of post-KT BMI and quantify the association between 1-year post-KT BMI change and long-term graft loss and mortality.

*Methods: Using SRTR, we estimated 5-year post-KT (n=151,170) BMI trajectories using an adjusted mixed effects model. We then estimated 1-year post-KT BMI change using unadjusted linear regression models defining BMI change by quartile [decrease (1^st quartile): change<-0.07 kg/m²/month; stable (2^nd quartile): -0.07≤change≤0.09 kg/m²/month; increase (3^rd and 4^th quartile): change>0.09 kg/m²/month]. We estimated mortality and graft loss risks by BMI change using adjusted Cox proportional hazards models.

*Results: BMIs increased in the first 3 years post-KT (0.65 kg/m²/year, 95% CI: 0.65, 0.66) and decreased in years 3-5 (-0.25 kg/m²/year, 95% CI: -0.26, -0.24). 1-year post-KT BMI decrease was associated with elevated risks of mortality (aHR=1.13, 95% CI: 1.10, 1.16) and graft loss (aHR=1.13, 95% CI: 1.10, 1.15) regardless of pre-KT obesity status. Among obese recipients (pre-KT BMI≥30kg/m²), BMI increase was associated higher mortality (aHR= 1.11, 95% CI: 1.06, 1.15) and graft loss (aHR=1.06, 95% CI: 1.02, 1.10) risks, relative to stable BMI. Among the non-obese, BMI increase was associated with lower graft loss (aHR=0.97, 95% CI: 0.95, 0.99) but not mortality risk (aHR= 0.99, 95% CI: 0.96, 1.01). The results remained robust in sensitivity analysis.

*Conclusions: BMI increases in the 3 years post-KT then decreases in years 3-5. BMI loss in all KT recipients and BMI gain in those with obesity should prompt more granular clinical risk assessments.

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