Background.Necrosis and apoptosis are different cell death mechanisms. Necrosis is a pathological process that occurs after destruction of the cell membrane. Apoptosis is a DNA-dependent cell death mechanism, which occurs under physiological and pathological conditions. Although necrosis is a well-defined phenomenon in an acute graft rejection, the occurrence and relevance of apoptosis during this process is largely unknown.Methods.The enterocyte apoptosis rates in allografted (n=24) and isografted (n=24) small intestines of the rat were compared using the in situ end-labeling technique.Results.In situ end-labeling showed a dramatically increased number of apoptotic enterocytes in allografted small intestines, whereas increased labeling could not be observed in isogeneic small intestinal grafts.Conclusions.We suggest that graft rejection-associated apoptosis, in addition to necrosis, plays an important role in the course of organ failure, and that the degree of apoptosis represents another reliable indicator for the diagnosis and prognosis of transplant rejection.