Despite the fact that CD4+ CD25+ Foxp3+ regulatory T cells (Treg cells) play a central role in maintaining self-tolerance and that IL-17-producing CD4+ T cells (Th17 cells) are pathogenic in many autoimmune diseases, evidence to date has indicated that Th17 cells are resistant to suppression by human Foxp3+ Treg cells. It was recently demonstrated that CD39, an ectonucleotidase which hydrolyzes ATP, is expressed on a subset of human natural Treg cells. We found that although both CD4+ CD25 high CD39+ and CD4+ CD25 high CD39− T cells suppressed proliferation and IFN-γ production by responder T cells, only the CD4+ CD25 high CD39+, which were predominantly FoxP3+, suppressed IL-17 production, whereas CD4+ CD25 high CD39− T cells produced IL-17. An examination of T cells from multiple sclerosis patients revealed a normal frequency of CD4+ CD25+ CD127 low FoxP3+, but interestingly a deficit in the relative frequency and the suppressive function of CD4+ CD25+ CD127 low FoxP3+ CD39+ Treg cells. The mechanism of suppression by CD39+ Treg cells appears to require cell contact and can be duplicated by adenosine, which is produced from ATP by the ectonucleotidases CD39 and CD73. Our findings suggest that CD4+ CD25+ Foxp3+ CD39+ Treg cells play an important role in constraining pathogenic Th17 cells and their reduction in multiple sclerosis patients might lead to an inability to control IL-17 mediated autoimmune inflammation.