Pancreatic islets are avascular immediately after transplantation. Although the islets are rapidly revascularized, it is uncertain whether the revascularization produces an adequate oxygenation of the transplanted islet tissue. We measured pO(2), blood flow and vascular density in mouse or human islets 1 month after transplantation to nude mice. Tissue pO(2) was measured with Clark microelectrodes. Blood perfusion was measured with laser-Doppler flow cytometry, whereas vascular density was determined in histological specimens stained for the lectin Bandeiraea simplicifolia (BS-1). Both the transplanted mouse and human islets had a pO(2) 15-20% of that in endogenous mouse islets. Moreover, the vascular density of the transplanted islets was decreased compared with that of endogenous mouse and human islets. Graft blood perfusion was approximately 50% of renal cortex blood flow. A negative correlation was found between donor age and blood perfusion of the human islet grafts. A similar correlation was seen between donor age and the total vascular density of these grafts. In conclusion, transplanted human islets had a markedly decreased vascular density and pO(2) compared with endogenous islets. This has potential implications for clinical islet transplantations, because poor vascular engraftment may significantly increase the number of islets needed to obtain insulin independence.