*Purpose: Simultaneous heart and kidney transplantation (SHKT) has increased steadily in United States Previously it was shown that SHKT is associated with benefits in overall and cardiac graft survival. The aim of this study was to analyze kidney allograft outcomes in combined SHKT.

*Methods: We evaluated patient and kidney allograft survival for SHKT and compared them to kidney transplant alone (KTA) using the United Network for Organ Sharing (UNOS) database from 2015-2020. We also analyzed the incidence and risk factors of kidney graft failure and delayed graft function (DGF) with inverse probability weighting methods.

*Results: The number of SHKT increased significantly during the study period (p<0.05). When compared to KTA, SHKT recipients are older (57 vs 54 years), male (78% vs 60%), white (49 vs 35%), not on hemodialysis (30% vs 91%) at time of transplant and had longer length of post-transplant hospital stay (21 vs. 5 days). After adjustment of recipients and donor factors, in SKT recipients when compared to KTA, incidence of DGF was same [adjusted Odds Ratio (aOR) (95%CI) 1.08 (0.94-1.24), p=0.28], risk of patient death and graft failure were higher [adjusted Hazard Ratio- aHR(95% CI) 1.70 (1.45-1.99); 1.49 (1.29-1.71), p<0.001, respectively], and risk of kidney primary non-function was significantly higher [aOR (95%CI) 3.52 (2.45-5.04), p<0.001]. Primary non-function of kidney graft (40%) was the most common cause of graft failure in SHKT recipients. Cumulative failure of renal graft in SHKT recipients was seen early, 30-days post-transplant, and was driven primarily by patient death. Most common causes of death in SHKT recipients were infection (20%) and multi-organ failure (16%), which were higher when compared to KTA recipients.

*Conclusions: Among SHKT recipients, there is a significant risk of lower patient survival and high rates of kidney graft failure which was driven by significant primary non-function and patient death. Our data suggests re-evaluation of performing SHKT in all eligible candidates, identification of modifiable risk factors to optimize early outcomes and supports setting up of safety net policy for kidney after heart transplantation.

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