Biological markers capable of predicting the clinical outcome of antithyroid drug therapy could be clinically useful in selecting the modality of treatment for Graves' disease, but at present they are unavailable. In the present study we prospectively explore the value of 22 different peripheral blood T, B and NK lymphocyte subsets to predict remission and relapse in a group of 42 Graves' disease patients. Eighteen patients were studied at diagnosis, before treatment, and 24 during antithyroid drug therapy. All cases were followed-up for at least one year after finishing an 18 month cycle of methimazole therapy. The combination of flow cytometry and 3- color immunofluorescence did not reveal significant differences in the distribution of the major peripheral blood T, B and NK cell subsets between the relapsed patients and those in remission, both in the groups studied at diagnosis and in those analyzed during the cycle of antithyroid drug therapy. In our search for a prognostic marker for relapse prediction we found that some lymphoid subpopulations such as total B cells, total NK and NK CD8+ cells showed high sensitivity (88-100%). In turn, other subsets such as TCD8+, total T and B cells expressing the CD25 antigen displayed high specificity (77-88%).