Almost all cases of hyperthyroidism in children result from Graves’ disease (GD). Recent studies have confirmed a significant role of T regulatory cells (Tregs) in the development of autoimmune diseases. However, the interactions between T cell responses and Treg proliferation in GD are still poorly understood. The aim of this study was to assess the proliferation of Treg cells (Tregs) and CD3+ T lymphocytes isolated from 50 children with GD before and after treatment with the thyreostatic drug methimazole (MMI). The proliferation rates, measured by methyl-3H-thymidyne incorporation, of CD3+ cells and Tregs stimulated with mitogen phorbol 12-myristate 13-acetate (PMA) were compared with those of unstimulated cells. The proliferation rates of both PMA-stimulated and unstimulated CD3+ cells prior to treatment with MMI were significantly higher than after treatment. Simultaneously, the proliferation rates of both PMA-stimulated and unstimulated Tregs were significantly lower before MMI treatment. Moreover, we observed higher cell proliferation rates of unstimulated and PMA-stimulated Tregs before the initiation of MMI therapy and after treatment in patients who had no relapse of hyperthyroidism. There was a positive correlation between the CD3+ cells proliferation rate before MMI treatment and fT3, as well as fT4 concentration in peripheral blood. The proliferation rates of CD3+ T cells before and after MMI treatment positively correlated with the TSI index. Thus, children suffering from Graves’ disease presented lower Tregs proliferative potential compared with CD3+ T cells. Cocultures of CD3+ T cells and Tregs showed that Tregs were not capable of efficiently inhibiting the proliferation of CD3+ T cells in GD patients. Conclusions. MMI treatment reduced the proliferative activity of CD3+ T cells in pediatric GD patients and increased the proliferation rate of Tregs. We suggest that Treg cells that are partly dysfunctional in GD disease are probably suppressed by CD3+ T cells and that methimazole exerts some immunomodulatory effects. (Folia Histochemica et Cytobiologica 2014, Vol. 52, No. 1, 69–77)