The serum soluble interleukin 2 receptor (sIL2R) was measured in 38 first visited patients with Graves' disease and 29 normal controls. The serum sIL2R in 17 patients with Graves' disease was determined after treatment with antithyroid drugs (propylthiouracil) for a short period (1.2 +/- 0.5 months). The serum sIL2R was measured by sandwich enzyme linked immunosorbent assay. The sIL2R was significantly higher in patients before (3.04 +/- 0.19 U/ml) and after treatment (2.56 +/- 0.41 U/ml) than in normal controls (2.20 +/- 0.27 U/ml, P < 0.01). The mean value of serum sIL2R in 17 patients after treatment (2.56 +/- 0.41 U/ml) was substantially decreased as compared with that before treatment (2.99 +/- 0.14 U/ml, P < 0.01). The serum level of sIL2R in pretreatment patients was correlated significantly with T3(r = 0.5032, P < 0.05), but was not obviously related to T4 or rT3. These findings suggest that the human lymphocytes in patients with Graves' disease were activated in vivo and that sIL2R may be an useful immunological indicator of disease activity.