Prolonged exposure to dialysis before transplantation and black ethnicity are known risk factors for acute rejection and graft loss in kidney transplant recipients. Because the strength of the primed antidonor T cell repertoire before transplantation also is associated with rejection and graft dysfunction, this study sought to determine whether hemodialysis (HD) vintage and/or black ethnicity affected donor-directed T cell immunity. An enzyme-linked immunosorbent spot (ELISPOT) assay was used to measure the frequency of peripheral T cells that expressed IFN-gamma in response to donor stimulator cells before transplantation in 100 kidney recipients. Acute rejection occurred in 38% of ELISPOT (+) patients versus 14% of ELISPOT (-) patients (P = 0.008). The median (HD) vintage was 46 mo (0 to 125 mo) in ELISPOT (+) patients versus 24 mo (0 to 276 mo) in ELISPOT (-) patients (P = 0.009). Black recipients had a greater median HD vintage (55 versus 14 mo in nonblack recipients; P < 0.001). Black recipients with less HD exposure had a low incidence of an ELISPOT (+) test, similar to nonblack recipients. Among variables examined, only HD vintage remained a significant positive correlate with an ELISPOT (+) result (odds ratio per year of HD 1.3; P = 0.003). These data suggest that the risk for developing cross-reactive antidonor T cell immunity increases with longer HD vintage, providing an explanation for the previously observed relationship between increased dialysis exposure and worse posttransplantation outcome. Longer HD vintage may also explain the increased T cell alloreactivity that previously was observed in black kidney recipients.