We developed a chimeric human-severe combined immunodeficient mouse model to study human allograft rejection. Mice received first partial thickness human skin grafts and then, after anastomosis of the mouse with graft human microvessels, human lymphocytes allogeneic to the skin. By 2 weeks, the skin grafts uniformly developed changes that resemble first-set skin rejection in humans. Vascular cell adhesion molecule 1 and major histocompatibility complex class II molecules were induced on the human vascular endothelium at day 6, prior to significant T-cell infiltration. Perivascular human CD4+ and CD8+ T-cell infiltrates were marked by day 11. Some T cells, adjacent to injured human vessels, expressed the cytolytic granule protein perforin. The human microvessels were destroyed by day 16 without significant damage to human keratinocytes or adjacent mouse microvessels. This small animal model may permit evaluation of potential therapeutic reagents that inhibit human T-cell-mediated injury.