Th17 cells are involved in the defense against bacteria and fungi and play a prominent role in the pathogenesis of autoimmune diseases, but research on human Th17 cells is hindered due to the lack of a surface marker. In this study, we report that a subset of human and mouse CD4(+) T cells as well as human Th17 T cell clones express IL-17A on their surface upon stimulation. Correlation of surface IL-17A expression with intracellular IL-17A production and with RORgammat mRNA expression identified surface IL-17A as a specific marker for human and mouse Th17 cells. Phenotype characterization of ex vivo CD4(+) IL-17A(+) cells showed that the chemokines CCR6 and CCR4, costimulatory molecules, as well as CD2 and CD49d were more prominently expressed on these cells than in surface IL-17A(-) cells, supporting the concept of Th17 cells as a potent inflammatory effector subtype. In addition, we generated human Th1, Th1/17 (producing both IFN-gamma and IL-17A), and Th17 T cell clones based on single cell sorting of surface IL-17A(-), IL-17A(int), and IL-17A(high) CD4(+) T cells, respectively, and showed the plasticity of the double producing clones to the cytokine milieu. The identification of surface IL-17A as a marker for Th17 cells should facilitate research on this subset.