*Purpose: The risk of significant weight mismatch between donors and recipients (D-R) undergoing living donor kidney transplant (LDKT) is not well established. Further, how this risk compares with deferring LDKT for a weight matched deceased donor kidney transplant (DDKT) is unknown. Therefore, the purpose of this study was to determine the risk of D-R weight mismatch in LDKT and how this relates to deferring LDKT to facilitate a weight matched DDKT.

*Methods: We used multivariable Cox proportional hazards models and the Scientific Registry of Transplant Recipients to determine the association of weight mismatched D-R (>50 kg, 30-50 kg or 10-30 kg ((DR); and <10 kg (D=R)) with death-censored graft failure in LDKT recipients in the USA from 2006-2017. We explored outcomes in weight mismatched LDKT D-R relative to weight matched (< 10 kg difference) DDKT D-R (separately for i. all deceased donors and ii. only standard criteria deceased donors (SCD)). Finally, we explored the impact of combined D-R weight-sex mismatch in LDKT versus weight and sex-matched DDKT.

*Results: In LDKT, the risk of graft loss was increased in the setting of extreme D50kg difference relative to D=R), but minimal if recipients were up to 30k larger than their donor (HR 1.059, 95% CI 0.977-1.149). However even in the highest risk LDKT D-R weight pairing, graft loss was still less than in weight-matched DDKT (HR 0.646, 95% CI 0.541-0.772 for >50 kg D50 kg D30 kg larger than their female donor, relative to weight-matched male donor-male recipient). This remained superior to weight-sex matched DDKT however.

*Conclusions: D-R weight mismatch is important in LDKT, however graft survival remains superior to proceeding with weight matched DDKT. The same is true for weight-sex mismatch in LDKT versus weight-sex matched DDKT. Optimizing D-R matching in LDKT could be facilitated through a national kidney paired donation registry.

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