Monoclonal antibodies reacting with cell surface antigens of helper (T4), suppressor (T8) T cells and common T-cell antigen (T3) were used by an immunofluorescence technique to enumerate peripheral T-lymphocytes in 37 patients suffering from euthyroid Graves' ophthalmopathy. According to the time when the disease appeared, the patients were divided into two groups: the first group in whom the disease was present for less than three years, and the second group, in whom the disease had lasted for more than five years. The most important finding was a decrease of OKT8+ cell-percentage in patients in whom the disease had lasted for less than three years. The results confirm the hypothesis made by Volpe that the ophthalmopathy can be considered as a new autoimmune organ-specific disease and that it must be considered to be separated from the autoimmune thyroid disease.