It has recently been shown that interleukin (IL)-21 is produced by Th17 cells, functions as an autocrine growth factor for Th17 cells, and plays critical roles in autoimmune diseases. In this study, we investigated the differentiation and characteristics of IL-21–producing CD4⁺ T cells by intracellular staining. Unexpectedly, we found that under Th17-polarizing conditions, the majority of IL-21–producing CD4⁺ T cells did not produce IL-17A and -17F. We also found that IL-6 and -21 potently induced the development of IL-21–producing CD4⁺ T cells without the induction of IL-4, IFN-γ, IL-17A, or IL-17F production. On the other hand, TGF-β inhibited IL-6– and IL-21–induced development of IL-21–producing CD4⁺ T cells. IL-2 enhanced the development of IL-21–producing CD4⁺ T cells under Th17-polarizing conditions. Finally, IL-21–producing CD4⁺ T cells exhibited a stable phenotype of IL-21 production in the presence of IL-6, but retained the potential to produce IL-4 under Th2-polarizing conditions and IL-17A under Th17-polarizing conditions. These results suggest that IL-21–producing CD4⁺ T cells exhibit distinct characteristics from Th17 cells and develop preferentially in an IL-6–rich environment devoid of TGF-β, and that IL-21 functions as an autocrine growth factor for IL-21–producing CD4⁺ T cells.