Graves' disease has attracted considerable attention as an autoimmune disease. In this study, cellular immunity in patients with this disease was assessed. Specifically examined was the lymphocyte response to mitogens, Interleukin-2 (IL-2) production from peripheral blood mononuclear cells (PBMCs) and the percentage of lymphocyte subsets. No significant difference was observed in the lymphocyte response to phytohemagglutinin (PHA), concanavalin A (Con A) and poke-weed mitogen (PWM) between untreated patients with Graves' disease and healthy people. IL-2 production in untreated patients, however, was significantly greater than that of healthy people. While a significant decrease was observed in the percentage of CD8+ cells in untreated patients, no difference was found in the percentage of CD5+, CD4+ and HLA-DR+ cells between them and healthy people. It is thought that the enhancement of IL-2 production by PBMCs and the decrease in the percentage of CD8+ cells (cytotoxic/suppressor cell) are associated with abnormalities in the immune system.