In human heart and kidney transplantations, granzyme B (GrB) and perforin have both been shown to be predictive markers for acute cellular rejection (ACR). We investigated the tissue expression and possible relationship of GrB and perforin to the clinical outcome, histopathology, and function of intestinal transplants. In 13 consecutive patients undergoing small intestine transplantation, histologic/immunohistochemical rejection monitoring was performed together with GrB and perforin immunostaining (score "0", 0%-10% positive lymphocytes; "1", 10%-25%; "2", 25%-50%; "3", >50%). Eleven patients are currently alive and well. All 11 had at least one episode of ACR: one patient had 6 episodes of severe ACR requiring retransplantation; the remaining 10 experienced only mild or moderate rejection. Both GrB and perforin were always co-expressed. A highly significant correlation was observed between GrB/perforin scores and histological severity of ACR (Pearson's coefficient, R < 0.0009). Interestingly, score 3 GrB/perforin immunostaining was recorded only in the context of severe ACR; all the histologically negative or "indeterminate" biopsies (n = 6) taken from a single affected patient showed GrB/perforin scores of 1 or 2. By contrast, none of the other tested histologically negative/"indeterminate" biopsies (n = 350), including those performed during graft stabilization, had raised GrB or perforin scores. We conclude that in intestinal transplantation recipients, a direct correlation seems to exist between histologically confirmed ACR and raised GrB/perforin immunohistochemical scores. Our findings suggest the need to investigate the possibility of predicting ACR by routine serum polymerase chain reaction (PCR) monitoring, which would reduce discomfort to patients.