Subsets of T cells have been identified by the expression of different isoforms of CD45. Some T cells expressing CD45RA (gp220) have been characterized as “naive” in their response to antigens including alloantigens, whereas T cells expressing CD45RO (gp180) have been characterized as “memory” T cells. To examine the potential association between kidney allograft rejection and infiltration with these subsets, immunohistologic labeling of serial frozen sections from 17 nonrejecting and 18 rejecting kidney transplant biopsies was performed. Biopsies were selected on the basis of unambiguous clinical pathological diagnosis, no antirejection therapy within 1 month before biopsy, and enough tissue available for immunohistologic labeling. Sections were labeled for CD3, CD4, CD8, CD45RA, and CD45RO, and slides were evaluated in a masked manner. The numbers of cells were counted in 10 (400×) high power fields (HPF) in each section. The medians of average cell counts were significantly higher in the rejecting compared with nonrejecting group for CD3+, CD4+, CD8+, CD45RA+, and CD45RO+ T cells in both diffuse and aggregate patterns of infiltration. However, the numbers of infiltrating cells per HPF in rejecting grafts were most striking for CD8+ and CD45RO+ cells: 16/18 rejecting vs. 1/17 nonrejecting cases had more than 5 CD8+ cells/HPF. Likewise, 17/18 rejecting vs. 1/17 nonrejecting cases had more than 5 CD45RO+cells/HPF. In both cases, the difference was most striking for cells infiltrating in a diffuse versus aggregate pattern. Significantly higher CD45RO+to CD45RA+ratios were observed in rejecting versus nonrejecting groups, both in the diffuse and aggregate patterns. Within the rejecting cases, the CD45RO+ to RA+ ratio was significantly higher in diffuse versus aggregate patterns of infiltration (P<0.0002). These results indicate that a significant increase in CD8+, CD45RO+, and the ratio of CD45RO+ to CD45RA+ T cells in a diffuse pattern of infiltration is most characteristic of clinical renal allograft rejection.