In the present study we analyzed some circulating lymphocyte subsets in eleven patients affected by Graves' disease before and after three and six months of methimazole treatment. Peripheral blood mononuclear cells were studied by a panel of monoclonal antibodies with single and double fluorescence cytometric analysis. Our results demonstrated an increased percentage of CD5+B cells and HLADR+T lymphocytes at the beginning of the disease in comparison to the normal controls (p < 0.001), and a significant decrease after six months of treatment (p < 0.01 and p < 0.05, respectively). The CD4+ CD45RA+ subset was significantly reduced in untreated Graves' patients in comparison to the normal group (p < 0.01), and increased towards normalization after six months of treatment. The significant modifications of lymphocyte subsets, as well as the reduction of thyroid autoantibodies, support a direct or mediated effect of methimazole on the immune system.