The present study was undertaken to examine the cellular control arm of the immune response with regard to T lymphocyte proliferation in euthyroid Graves' ophthalmopathy. Twenty patients with euthyroid Graves' ophthalmopathy (7 on antithyroid drugs and 13 on no treatment) and 18 healthy controls were studied in an infection-free period. Mitogen-stimulated cellular interleukin 2 (IL2) receptor expression, soluble interleukin 2 receptor release, and interleukin 2 production, were studied in peripheral blood mononuclear cells cultured for 24 h. The cellular IL2 receptor expression and soluble IL2 receptor release did not differ between the patients and healthy controls. In contrast, IL2 production in response to pokeweed mitogen stimulation was increased in lymphocytes from patients with Graves' ophthalmopathy. The IL2 release did not correlate with the quantities of cellular and soluble IL2 receptor. The mitogen-stimulated cellular IL2 receptor expression, IL2 receptor release, and IL2 production did not differ between patients with or without carbimazole therapy. Despite a suggested role of autoreactive T cells in mediating the development and propagation of autoimmune thyroid disease, this study fails to demonstrate a defective T lymphocyte activation state in patients with Graves' ophthalmopathy during an euthyroid state.