Tubulointerstitial fibrosis is a common consequence of a diverse range of kidney diseases that lead to end-stage renal failure. The degree of fibrosis is related to leukocyte infiltration. Here, we determined the role of different T cell populations on renal fibrosis in the well-characterized mouse model of unilateral ureteric obstruction. Depletion of CD4(+) T cells in wild-type mice with a monoclonal antibody significantly reduced the amount of interstitial expansion and collagen deposition after 2 weeks of obstruction. Reconstitution of lymphopenic RAG knockout mice with purified CD4(+) but not CD8(+) T cells, prior to ureteric obstruction, resulted in a significant increase in interstitial expansion and collagen deposition. Wild-type mice had significantly greater interstitial expansion and collagen deposition compared with lymphopenic RAG(-/-) mice, following ureteric obstruction; however, macrophage infiltration was equivalent in all groups. Thus, our results suggest that renal injury with subsequent fibrosis is likely to be a multifactorial process, with different arms of the immune system involved at different stages. In this ureteric obstruction model, we found a critical role for CD4(+) T cells in kidney fibrosis. These cells could be a potential target of therapeutic intervention to prevent excessive fibrosis and loss of function due to renal injury.