Interest in T-lymphocyte subsets has arisen because of their involvement in the autoimmune process. Contradictory results have been published in the literature about the number of peripheral blood lymphocyte subsets in autoimmune diseases. In order to investigate the number and distribution of peripheral blood lymphocyte subsets in autoimmune thyroid disease, the levels of total T-lymphocytes (CD3), T-helper (CD4) and T-suppressor/cytotoxic (CD8) lymphocytes were determined in 44 patients with Graves' disease (1), multinodular goiter (2) and Hashimoto's thyroiditis (3). All patients had high levels of antithyroglobulin and thyroid antiperoxidase (antimicrosomal) antibodies. The T subset levels were related to the functional thyroid status, measured as serum free thyroxine (FT4) and thyrotropin (TSH). Our data show the existence of a strong influence of functional status on CD3, CD4 and CD8 levels, as reflected in the significant correlations obtained with FT4 (negative) and TSH (positive). A significant decrease in all populations was observed in Graves' disease hyperthyroid patients. A decrease in the CD4/CD8 ratio in Hashimoto's thyroiditis hypothyroid patients was observed, in contrast to an increase in the ratio in autoimmune hyperthyroid patients. This points to the CD4/CD8 ratio as a differential characteristic between the two autoimmune (hypothyroid and hyperthyroid) entities, independent of free thyroxine levels. No significant correlation was found between antithyroid antibody levels and peripheral blood T-lymphocyte subsets or serum levels of FT4 and TSH.