*Purpose: The aim of this study was to describe the changes in body mass index (BMI) after kidney transplantation and assess how this influences long-term outcomes.

*Methods: Data were collected for all kidney transplant recipients between January 2007 and July 2016. Changes in BMI over the post-transplant period were modelled using a generalised estimating equation. The change in BMI from pre-transplantation to six months was then calculated for each patient. These were categorised into three groups: stable BMI (a change of ±1.5 kg/m²), BMI reduction and BMI increase (changes of >1.5 kg/m²), between which a range of outcomes were compared.

*Results: Data was available for 1,344 patients, who had a geometric mean pre-transplant BMI of 27.3 kg/m². This declined significantly (P<0.001), to a geometric mean of 25.6 kg/m² one month after transplantation, before increasing and stabilising to pre-transplant levels by 36 months (geometric mean 27.2 kg/m2, P=0.522) (Figure 1). The n=882 patients with BMI measurements at six months, were divided into groups of reduced (n=303), stable (n=388) and increased (n=131) BMI, relative to pre-transplantation levels. On multivariate analysis, 12-month creatinine levels were significantly higher in the BMI reduction cohort, with adjusted levels of 160.6 μmol/l, compared to 135.0 μmol/l in stable BMI. However, no significant associations were detected between six-month BMI change and patient survival, graft survival, incidence of post-transplant diabetes, cancer, or a range of clinical and histological outcomes (all P>0.05).

*Conclusions: Our data demonstrates that BMI significantly reduces in the first month after kidney transplantation, before increasing to pre-transplant levels at 3-5 years. Furthermore, patients with decreasing BMI at six-months have impaired graft function in the long-term. These observations conflict with the existing literature and warrant further investigation

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