Granulomatous with polyangiitis (GPA) is a type of ANCAassociated vasculitis. The etiology of the disease is complex and remains the subject of research. Recently, special attention has been paid to the role of Th17 and T regulatory lymphocytes (Treg). The main tool for GPA activity assessment has been the Birmingham Vasculitis Activity Score (BVAS). The aim of the study was to look for potential new markers of GPA activity among T lymphocyte subpopulation percentages in following groups: active GPA (BVAS> 2, n=19); after 6 months of treatment; in sustained GPA remission (BVAS = 0, n=18) and in the control group (n=24). Patients with exacerbation of GPA were re-sampled after half a year of immunosuppression. The lymphocyte immunophenotype was assessed by flow cytometry based on chemokine receptor expression and cytokine production after PMA and ionomycin stimulation. Treg lymphocytes were identified as CD25hiCD127low or FoxP3 + cells (transcription factor). GPA flare was associated with Treg cells depletion. Stable remission was characterized by higher Treg cells percentages than other groups. Expansion of Th17 cells was present in GPA patients regardless of disease activity. Immunophenotype analysis after 6 months of treatment, despite BVAS 0, showed no difference in any of the parameters analyzed, demonstrating a deep cellular abnormalities in the acute phase of GPA and high “inertia” of the disease process. It can be assumed that the percentage of Treg could be a marker of GPA activity.