Background: Although cardiac surgery among renal allograft recipients is relatively safe, less is known about the impact of cardiac surgery on the functioning renal allograft. This study assessed postoperative renal failure among renal transplant recipients undergoing cardiac surgery. Methods: The study population was identified by matching medical record numbers from the United Network for Organ Sharing Kidney Transplant Database to a cardiovascular surgery database and The Society of Thoracic Surgeons Adult Cardiac Surgery Database for the authors’ institution from January 1992 through August 2018. Results: One hundred seventy-nine renal transplant recipients with a functioning allograft underwent cardiac surgery a mean of 6.4 ± 5.6 years after renal transplantation. Thirty (17.6%) of the 170 patients either died or had allograft failure during the first postoperative year. Receiver-operating characteristics curve analysis using Cox regression demonstrated an optimal cutoff point for preoperative serum creatinine predicting postoperative allograft loss is 1.9 mg/dL (hazard ratio 3; 95% confidence interval, 1.5 to 6.9) with a model C statistic of 0.642. Conclusions: The current study affirms findings in the literature that cardiac surgery in renal transplant recipients carries acceptable perioperative morbidity and mortality. Renal transplant recipients who underwent cardiac surgery had a constant hazard of renal allograft loss similar to that of the general transplant population. A preoperative serum creatinine value greater than 1.9 mg/dL increases the risk for long-term renal allograft loss after cardiac surgery.