The present study was performed to elucidate the relationship between CTLA-4/CD28 molecules and stimulating (TSAb) and blocking (TBAb) antibodies to the TSH-receptor (TSH-R) in Graves’ disease. CD28 and CD152 (CTLA-4) are glycoprotein molecules which provide a potent costimulatory signal for T-cell activation and proliferation via interactions with their ligands, B7.1/B7.2 molecules, which are present on the surface of antigen-presenting cells. The aim of the study was to estimate the expression of cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4, CD152), CD28, B7.1 (CD80), and B7.2 (CD86) molecules on peripheral blood cells in patients with Graves’ disease (GD) (n = 55, mean age 15.5 ± 5.1 years) and nontoxic nodular goiter (NTNG) (n = 55, mean age 15.2 ± 4.5 years), in comparison with sex and age-matched healthy control subjects (n = 55, mean age 15.2 ± 3.9 years). The expression of the costimulatory molecules on mononuclear cells was analyzed by three-color flow cytometry using a Coulter EPICS XL cytometer. Detection of TSAb and TBAb to the TSH-R using JPO9 CHO cells in unfractionated serum was measured by a highly sensitive commercial radioimmunoassay. When compared with healthy control subjects and euthyroid patients with GD, untreated patients with GD showed a significant increase of CD152+ (p < 0.001, p < 0.001) and CD28+ (p < 0.01, NS) T lymphocytes, respectively. After 6–12 months of methimazole therapy, the percentage of these cells in the peripheral blood of hyperthyroid patients returned to normal values. In addition, patients with GD showed an increase in the percentage of both B7.1 (3.8%) and B7.2 (18.4%) molecules on activated monocytes, compared to patients with NTNG (0.5% p < 0.05, 2.5% p < 0.01, respectively) and healthy control subjects (0.2% p < 0.05, 0.8% p < 0.003, respectively). In patients with untreated GD there was a statistically significant positive correlation between the expression of CTLA-4 on the surface of peripheral blood T cells and the index of TSAb antibodies (R = 0.54, p < 0.001) as well as a negative correlation with TBAb antibody titer (R = –0.58, p < 0.001). However, no such correlations were noted with regard to CD28 and anti-TPO, anti-TG, and TRAb antibodies. We conclude that changes in the expression of costimulatory molecules on the surface of peripheral blood T cells and their significant relationship with the level of antithyroid antibodies indicate an involvement of these molecules in the pathogenesis of GD.