*Purpose: Transplanting kidneys small for the recipient’s size results in inferior renal graft function. Body surface area (BSA) is related to kidney size. We used the BSA index (BSAi) (Donor BSA/Recipient BSA) to assess whether the renal parenchymal mass provided by the donor is sufficient for the recipient.

*Methods: We included 26223 adult single kidney-only transplants (01/01/2007-31/12/2019) from the UK Transplant Registry. We divided renal transplants in groups: BSAi≤0.75, 0.751.25. We compared delayed graft function (DGF) rates, primary non-function (PNF) rates and graft survival among them in the entire cohort and for each donor type separately [living, donation after brain death (DBD), donation after circulatory death (DCD)] (reference category: BSAi≤0.75).

*Results: Cases with BSAi≤0.75 had the highest DGF rates in living-donor renal transplants (11.1%) (0.751.25: OR=0.318, 95%CI: 0.132-0.766, p=0.011) and DBD renal transplants (26.2%) (0.751.25: OR=0.65, 95%CI: 0.468-0.903, p=0.01). There were no significant differences in DCD renal transplants regarding DGF rates (just above 40% in all groups). No significant differences were found concerning PNF. Graft survival was similar among BSAi groups in living-donor and DBD renal transplants. DCD renal transplants with BSAi≤0.75 had shorter graft survival than the other groups (0.751.25: HR=0.45, 95%CI: 0.307-0.66, p<0.001). 5-year and 10-year graft survival rates were 73% and 58%, respectively, for DCD renal transplants with BSAi≤0.75.

*Conclusions: DGF risk is higher in living-donor and DBD renal transplants with BSAi≤0.75. Graft survival is greatly reduced in DCD renal transplants with BSAi≤0.75.

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