Although active inflammation may be deleterious and indicate immunologic activation in chronically rejected grafts, the underlying mechanism of tissue destruction has been little studied. Twenty-four cases of chronic rejection (CR) with or without acute rejection (AR) were stained with antibodies against CD3, CD8, CD68, granzyme B and TIA-1, and the number of positive cells were counted. Eleven cases of AR served as controls. The number of CD3 and CD8 positive cells increased in the acute on CR group compared to the CR group. About a half of CD3 positive T cells were CD8 positive in both groups, however, the proportion of TIA-1 or granzyme B positive cells was higher in the acute on CR group. The numbers of CD3, CD68, granzyme B and TIA-1 positive cells were higher in the AR group than the acute on CR group, however, no significant difference was found between the two groups. Serum creatinine level and proteinuria at the time of biopsy and the percentages of late onset AR and graft failure rate were higher in the acute on CR group than the CR group. Summarizing, these results suggest that infiltration of activated T cells containing cytotoxic granules plays a role in graft destruction in acute on CR.