Considerable indirect evidence suggests that T lymphocytes have a role in the pathogenesis of psoriasis. The goal of this study was to directly test the ability of T cells to maintain psoriasis pathology. Psoriatic skin was transplanted onto SCID mice, which were then injected with autologous T cells. T cells were cultured from either psoriatic skin lesions or peripheral blood and injected intradermally or intravenously. Control SCID mice transplanted with psoriasis grafts were not injected with T cells. After 10 wk, control psoriatic skin grafts not injected with T cells lost many of the features of psoriasis. Injection of peripheral blood T cells was not able to maintain these psoriatic features. In contrast, the injection of T cells derived from psoriatic skin was able to maintain the psoriatic phenotype. Psoriatic features that were maintained included epidermal thickness and labeling index and expression of HLA-DR, involucrin, and ICAM-1, as well as loss of expression of filaggrin. Injection of skin infiltrating T cells into skin of normal donors on SCID mice did not induce changes of psoriasis. The ability of T cells from lesional skin, but not peripheral blood, to maintain psoriasis suggests that psoriasis is mediated by an autoantigen reactive T cell, which is present at a higher frequency in the psoriatic lesion.