Granulomatosis with Polyangiitis (GPA) (formerly known as Wegener's granulomatosis) is a vasculitis of unknown etiology affecting predominantly small- to medium-sized vessels, usually involving the upper and lower respiratory tract and kidneys. Anti-neutrophil cytoplasmic autoantibodies are probably the initial cause of the inflammatory process that leads to the typical necrotizing lesions. In this issue of the European Journal of Immunology, Szczeklik etal. [Eur. J. Immunol. 2017. 47: 724-733] report some interesting findings on the possible involvement of T-cell subsets in the pathogenesis of the disease. This prospective study, performed on a large cohort of patients, identifies Th17 lymphocytes as the possible pathogenic subset of GPA, and Treg cells as the possible suppressors of the inflammatory process. These two subsets in peripheral blood could be used as cellular biomarkers of disease activity, and this would result particularly useful in the follow-up of patients once the immunosuppressive treatment has been initiated.