The aim of the present study was to determine the proportion of gamma/delta T-lymphocytes and CD16/CD56 (CD3- and CD3+) cells in the peripheral blood of children and adolescents with Graves' disease (GD; n = 27; mean age, 15.5 +/- 5.1 y) and nontoxic nodular goiter (NTNG; n = 25; mean age, 15.2 +/- 5.7 y), in comparison with sex- and age-matched healthy control subjects (n = 25; mean age, 15.9 +/- 2.4 y). In addition, in patients with GD, we investigated the effect of methimazole therapy on the proportion of these cells. We also looked for associations among the parameters investigated. The percentages of gamma/delta TCR+CD3+ lymphocytes and CD3+, CD16/56+CD3+, and natural killer (NK) cells were analyzed by the three-color flow cytometry using a Coulter EPICS XL cytometer. In patients with untreated GD, we observed a significant decrease in gamma/delta T (CD3+) (p < 0.002), CD16/56(CD3+) (p < 0.001), and NK (p < 0.001) cells in comparison with the healthy control subjects. After 2-6 mo of methimazole therapy, the percentages of gamma/delta TCR+CD3+ and CD16/56(CD3+) cells in peripheral blood of hyperthyroid patients returned to the normal values, whereas the percentages of NK cells normalized after 18-24 mo of therapy. These abnormalities were absent in children and adolescents with NTNG. Furthermore, there was no difference in the percentage of CD3+ lymphocytes in all of the groups. In the patients with untreated GD, we found a negative correlation between free thyroxine concentration in blood serum and the percentages of CD16/56 (CD3-) and gamma delta T cells (r = -0.5, p < 0.035; r = -0.4, p < 0.02). No such correlation was detected in patients with NTNG. We conclude that the abnormal distribution of CD16/CD56 (CD3- and CD3+) cells and gamma/delta T lymphocytes in the peripheral blood in children and adolescents with untreated GD suggests their role in the development of autoimmunity.