A large proportion of islets are lost after transplantation partly due to a lack of functional vasculature. Islets revascularize from host tissue but the process takes up to 2 weeks and has been suggested to result in reduced vascular density in engrafted islets. We describe a method for observing and quantifying the revascularization of intraportally transplanted islets that includes number, density, and branching of islet capillaries. Syngeneic islets were transplanted selectively into the two right posterior lobes of the liver of adult Lewis rats. Sections of the livers were dual stained for insulin and Bandeiraea simplicifolia and analyzed for islet morphology, area, and vascular density from day 0 to day 14 posttransplant and compared to native islets. Vascular density was 1431 +/- 75.7 vessels/mm2 in native islets and fell to 325.3 +/- 30.8 vessels/mm2 (p < 0.001) by day 1 posttransplant and subsequently increased until day 14 when it was significantly higher than in native islets (2612.5 +/- 107.8 vessels/mm2, p < 0.001). The percentage of islet area occupied by vascular space was 9.1 +/- 0.9% in native islets. After falling to 2.3 +/- 0.3% (p < 0.001) 1 day posttransplant this rose to supranormal levels (21.5 +/- 0.8%, p < 0.001) by day 14. The index of capillary branching was 0.771 +/- 0.017 in native islets and fell to 0.465 +/- 0.02 (p = 0.001) by day 3 but returned to native values by day 7 posttransplantation (0.726 +/- 0.03). This technique provides a robust method for tracking and quantifying the revascularization of intraportally transplanted islets, which should enable the comparison of different strategies aimed at accelerating islet revascularization.