In pancreatic islet transplantation, the adhesion of activated leukocytes to endothelial cells and the loss of microvascular integrity represent the critical microcirculatory events, which promote loss of graft function due to rejection. With the view that oxygen radicals may contribute to graft rejection, we studied the effect of the antioxidant vitamin E on microvascular rejection of islet grafts. Islets were transplanted syngeneically and xenogeneically (rat) into dorsal skin-fold chambers of hamsters, which received a non-vitamin-E-supplemented laboratory chow. Treated animals with xenografts were fed with a diet supplemented with vitamin E in a low (150 mg/kg) and high (8000 mg/kg) concentration. Intravital fluorescence microscopy demonstrated complete vascularization of syngeneic grafts at day 10 after transplantation, intact islet microcirculation at day 20 with a functional capillary density of 653 +/- 6 cm-1, and only few leukocytes adherent to the endothelial lining of the islets' microvasculature (88 +/- 23 mm-2). Xenogeneic islets showed initial signs of rejection at day 6, including adhesion of leukocytes to the microvascular endothelium (610 +/- 110 mm-2) and loss of endothelial integrity. After 20 days, functional capillary density was significantly lower (173 +/- 68 cm-1) when compared with syngeneic grafts, indicating failure of graft acceptance. Supplementation of the diet with low and high concentrations of vitamin E resulted in a significant (P < 0.05) reduction of xenograft leukocyte-endothelium interaction (146 +/- 29 mm-2 and 109 +/- 42 mm-2) at day 6 after transplantation and and adequate development of functional capillary density at day 20 (478 +/- 36 cm-1 and 539 +/- 86 cm-1; P < 0.05), indicating prevention of microvascular rejection. We conclude that dietary supplementation of the lipophilic antioxidant vitamin E attenuates leukocyte-endothelial cell interactions, preserves microvascular integrity, and thus inhibits microvascular rejection in a dose-dependent fashion. Our study underscores the pivotal mediator role of reactive oxygen species in islet xenograft rejection and, furthermore, suggests that dietary vitamin E may act as an adjunct anti-rejection treatment in clinical islet transplantation.