There is an increasing evidence that CD3+ cells, bearing gammadelta T cell receptors representing a minor subpopulation of T cells in the peripheral blood of humans are involved in the development of autoimmunity. The aim of the present study was determination of the gammadelta T cell subpopulation levels in the peripheral blood of subjects with Graves' disease and newly diagnosed type 1 diabetes in comparison to age-matched healthy controls. The percentages of CD3+, CD8+, gammadelta TCR+CD8+, gammadelta TCR+CD8- lymphocyte subsets were measured by flow cytometry. In the peripheral blood of newly diagnosed Graves' disease patients we showed a significant decrease of gammadelta TCR+ cells and gammadelta TCR+CD8- subset content in comparison to the percentages observed in subjects after methimazole treatment and in healthy controls. We also found a significant increase of gammadelta TCR+CD8+ cells in the peripheral blood of subjects with insulin-dependent diabetes, treated with insulin for 3-6 months. The present findings confirm our previous hypothesis that gammadelta TCR+CD8+ lymphocyte subset could play a role in the pathogenesis of diabetes type 1, probably as regulatory T cells and could be induced by delivery of exogenous insulin. Our results suggest that gammadelta T cells (gammadelta TCR+CD8- subset) could also play an important role in the development of Graves' disease and that their levels are modulated by thyreostatic treatment.