CTLA-4 and CD28+ are regulators of T cell activation. The CTLA-4 gene is associated with variety of autoimmune diseases. The aim of the study was to evaluate changes in basic T cell subpopulations, and the expression of CD152+ and CD28+ before and after T cell stimulation in children with autoimmune thyroiditis (AT), as compared with control subjects. Blood samples were obtained from 35 AT children and 25 healthy children. CD markers were evaluated by flow cytometry at baseline, after the culture with phytohemagglutinin and without stimulation. At baseline, CD152+ expression was lower in patients than in controls (P<10(-6)). After stimulation, there were an increase in CD152+ T cells and decreases in CD28+ and CD4+ cells in controls (P<0.01). In AT children, CD152+ T cells remained stable. CD4+CD152+ T cells correlated inversely with antithyroglobulin antibodies. We conclude that alterations in lymphocyte markers are associated with AT. Stimulation leads to differing changes in T-lymphocyte subsets in both examined children populations.