CD4(+) T cells play a pivotal role in the pathogenesis of autoimmune disease, including human and experimental crescentic GN. Micro-RNAs (miRs) have emerged as important regulators of immune cell development, but the impact of miRs on the regulation of the CD4(+) T cell immune response remains to be fully clarified. Here, we report that miR-155 expression is upregulated in the kidneys of patients with ANCA-associated crescentic GN and a murine model of crescentic GN (nephrotoxic nephritis). To elucidate the potential role of miR-155 in T cell-mediated inflammation, nephritis was induced in miR-155(-/-) and wild-type mice. The systemic and renal nephritogenic TH17 immune response decreased markedly in nephritic miR-155(-/-) mice. Consistent with this finding, miR-155-deficient mice developed less severe nephritis, with reduced histologic and functional injury. Adoptive transfer of miR-155(-/-) and wild-type CD4(+) T cells into nephritic recombination activating gene 1-deficient (Rag-1(-/-)) mice showed the T cell-intrinsic importance of miR-155 for the stability of pathogenic TH17 immunity. These findings indicate that miR-155 drives the TH17 immune response and tissue injury in experimental crescentic GN and show that miR-155 is a potential therapeutic target in TH17-mediated diseases.