T cell functional differentiation is mediated by lineage-specific transcription factors. T helper 17 (Th17) has been recently identified as a distinct Th lineage mediating tissue inflammation. Retinoic acid receptor-related orphan receptor γ (RORγ) was shown to regulate Th17 differentiation; RORγ deficiency, however, did not completely abolish Th17 cytokine expression. Here, we report Th17 cells highly expressed another related nuclear receptor, RORα, induced by transforming growth factor-β and interleukin-6 (IL-6), which is dependent on signal transducer and activator of transcription 3. Overexpression of RORα promoted Th17 differentiation, possibly through the conserved noncoding sequence 2 in Il17-Il17f locus. RORα deficiency resulted in reduced IL-17 expression in vitro and in vivo. Furthermore, RORα and RORγ coexpression synergistically led to greater Th17 differentiation. Double deficiencies in RORα and RORγ globally impaired Th17 generation and completely protected mice against experimental autoimmune encephalomyelitis. Therefore, Th17 differentiation is directed by two lineage-specific nuclear receptors, RORα and RORγ. © 2008 Elsevier Inc. All rights reserved.